A modifier is a word or a group of words that provide you with more information about a word or noun in a sentence. Modifiers can be adjectives, adverbs or phrases. They give sentences the exactness and accuracy of expression or detail that would otherwise be lacking. With the use of modifiers, we know how fast a duck walks, how fierce that dog is and how colourful the flowers on display are.

As mentioned, a modifier can be:
an adjective: All her friends know she has a nasty temper.
an adverb: I smiled sweetly at her, but she didn't smile back.
a phrase: How many of you love that monkey with the longest tail in the zoo?