What is a modifier? how many types of modifiers are there?

A modifier is a word, phrase, or clause which functions as an adjective or an adverb to describe a word or make its meaning more specific.

Modifier: a modifier is an optional element in phrase structure or clause structure. A modifier is so called because it is said to modify (change the meaning of) another element in the structure, on which it is dependent. Typically the modifier can be removed without affecting the grammar of the sentence.