Adjective: In grammar, a word used with a noun, to express the quality of the noun.  An adjective gives attributes to the noun.  It limits, defines, or specifies the noun.  Thus, in the phrase, a wise ruler, wise is the adjective.  

I am sorry.  If you are an American, you may not know what a wise ruler is.  Let me give you some other examples:

  • fat cat
  • snowy mountains
  • hot coffee
  • fast car

“You use adjectives to give your nouns a little attitude or to communicate clearly. Without these important parts of speech, the president would live in The House, it would be difficult to give someone directions to the store, and there would only be one size available at Starbucks.”  (vocabulary.com)

The word adjective comes from two Latin words.  Ad means: to, toward, or next to.  Iacio, means: I throw.  An adjective, then, is an extra word, thrown next to a noun to give the noun a little something extra.