2 Answers

It really would have been easier to explain if I could have used pictures but  read below  

A fraction is a "portion " of a whole thing   example-   1/2 of  1 apple that's cut into 4 pieces  gives you 2 pieces

now cut the same apple into 8 pieces and 1/2 would give you 4 pieces because 4 is 1/2 of 8 but we "simplify " ( break it down into its simplest form  using smaller numbers but meaning the same amount)  we wouldnt say we have 4/8 or 8/16 because no matter how many pieces you cut that apple into 1/2 is 1/2   

to do this you find a number that will devide into both numbers like 7/21     seven devides into itself 1 time and 21 can be devides by 7  three times so 7/21 can be simplified. to 1/3 .If BOTH numbers do not have a common denominator ( a number that can equally go into another number without any remainders)   than it is already simplified to its lowest fraction. 

Just remember that BOTH numbers have to be able to be divided by the same number.to "break it down"  as we use to call or "simplify" it.