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In grammar, an adjective is a 'describing' word; the main syntactic role of which is to qualify a noun or noun phrase, giving more information about the object signified.
Adjectives are one of the traditional eight English parts of speech, although linguists today distinguish adjectives from words such as determiners that formerly were considered to be adjectives. In this paragraph, "traditional" is an adjective, and in the preceding paragraph, "main" is.

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An adjective is a word that modifes or describes a noun

Adjectives can often be identified in English by their position before a noun, i.e. 'the old barn'. Examples include 'green', 'intelligent' or 'interesting'.