3 Answers

 After Cain killed his brother Abel, God declared to Cain, "Now you are under a curse and driven from the ground, which opened its mouth to receive your brother's blood from your hand. When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth" (Genesis 4:11-12). In response, Cain lamented, "My punishment is more than I can bear. Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me" (Genesis 4:13-14). God responded, "Not so; if anyone kills Cain, he will suffer vengeance seven times over." Then the Lord put a mark on Cain so that no one who found him would kill him" (Genesis 4:15-16). What was this "mark" that God put on Cain to prevent vengeance from being taken against Cain?

The nature of the mark on Cain has been the subject of much debate and speculation. The Hebrew word translated "mark" is 'owth and refers to a mark, sign, or token. Elsewhere in the Hebrew Scriptures, 'owth is used 79 times and is most frequently translated as "sign." So, the Hebrew word does not identify the exact nature of the mark God put on Cain. Whatever it was, it was a sign/indicator that Cain was not to be killed. Some propose that the mark was a scar, or some kind of tattoo. Whatever the case, the precise nature of the mark is not the focus of the passage. The focus of the passage is that God would not allow people to take vengeance against Cain. Whatever the mark on Cain was, it served this purpose.

In the past, many believed the mark on Cain to be dark skin, that God changed the color of Cain's skin to black in order to identify him. With the corresponding curse that Cain received, the belief that the mark was black skin caused many to believe that people of black skin were cursed. Many used the mark of Cain as an excuse for the African slave trade and discrimination against people with black/dark skin. This interpretation of the mark of Cain is completely unbiblical. Nowhere in the Hebrew Scriptures is 'owth used to refer to skin color. The curse on Cain in Genesis chapter 4 was on Cain himself. Nothing is said of Cain's curse being passed on to his descendants. There is absolutely no biblical basis to claim that Cain's descendants had dark skin. Further, unless one of Noah's sons' wives was a descendant of Cain (possible but unlikely), Cain's line was terminated by the Flood.

What was the mark that God put on Cain? The Bible does not specifically say. The meaning of the mark, that Cain was not to be killed, was more important than the nature of the mark itself. Whatever the mark on Cain was, it had no connection to skin color or a curse on the descendants of Cain. To use the mark on Cain as an excuse for racism and discrimination is blatantly and absolutely unbiblical.

After Cain killed his brother Abel, God declared to Cain, "Now you are under a curse and driven from the ground, which opened its mouth to receive your brother's blood from your hand. When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth" (Genesis 4:11-12). In response, Cain lamented, "My punishment is more than I can bear. Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me" (Genesis 4:13-14). God responded, "Not so; if anyone kills Cain, he will suffer vengeance seven times over." Then the Lord put a mark on Cain so that no one who found him would kill him" (Genesis 4:15-16). What was this "mark" that God put on Cain to prevent vengeance from being taken against Cain?

The nature of the mark on Cain has been the subject of much debate and speculation. The Hebrew word translated "mark" is 'owth and refers to a mark, sign, or token. Elsewhere in the Hebrew Scriptures, 'owth is used 79 times and is most frequently translated as "sign." So, the Hebrew word does not identify the exact nature of the mark God put on Cain. Whatever it was, it was a sign/indicator that Cain was not to be killed. Some propose that the mark was a scar, or some kind of tattoo. Whatever the case, the precise nature of the mark is not the focus of the passage. The focus of the passage is that God would not allow people to take vengeance against Cain. Whatever the mark on Cain was, it served this purpose.
In the past, many believed the mark on Cain to be dark skin, that God changed the color of Cain's skin to black in order to identify him. With the corresponding curse that Cain received, the belief that the mark was black skin caused many to believe that people of black skin were cursed. Many used the mark of Cain as an excuse for the African slave trade and discrimination against people with black/dark skin. This interpretation of the mark of Cain is completely unbiblical. Nowhere in the Hebrew Scriptures is 'owth used to refer to skin color. The curse on Cain in Genesis chapter 4 was on Cain himself. Nothing is said of Cain's curse being passed on to his descendants. There is absolutely no biblical basis to claim that Cain's descendants had dark skin. Further, unless one of Noah's sons' wives was a descendant of Cain (possible but unlikely), Cain's line was terminated by the Flood.
What was the mark that God put on Cain? The Bible does not specifically say. The meaning of the mark, that Cain was not to be killed, was more important than the nature of the mark itself. Whatever the mark on Cain was, it had no connection to skin color or a curse on the descendants of Cain. To use the mark on Cain as an excuse for racism and discrimination is blatantly and absolutely unbiblical.

From > http://www.gotquestions.org/mark-Cain.html

Hi Bub I would liken it to, why the Police fasten Computerised tags to some  trublesome type  of human, one its for their own protection, also its for the protection of the civilians within the community were he or she would roam.

I guess his mark was like a tag so  God and everyone else could keep an eye on him.