The word “modifier” is derived from the verb “modify.” In grammar, when a word “modifies” another word, it adds to its meaning so that it becomes clearer and more fully-described–a dog is not just a dog, for example, it’s a small, white, fluffy dog. A pepper isn’t just spicy; it’s incredibly spicy.
A “modifier” is a word that refines or adds information or description to the meaning of another word, phrase, or sentence. The most typical modifiers are adjectives and adverbs. Adjectives describe nouns (“soft fur”; “excellent progress”). Adverbs describe adjectives (“extremely hot”; “very disappointing”) or verbs (“walked quickly”; “immediately refused”). Adverbs also modify phases (“completely in the dark”) and whole statements (“Unfortunately, we can’t go on vacation this year”). Nouns can often be used as modifiers of other nouns “(laboratory tests”; “cancer research”). Present and part participles of verbs are often used as modifiers too (“He arrived at my door, breathing hard and sweating”; “She finally returned my bowl, chipped in two places”), and sometimes participles become considered adjective modifiers in their own right (“a boring lecture”; “a broken promise”).
Prepositional phrases can be modifiers too. If we say “the man in the blue shirt,” the prepositional phrase “in the blue shirt” makes it clear which man we’re talking about. Similarly, relative clauses are modifiers too. If we say “the neighbor who lives upstairs,” the relative clause tells us which neighbor we are referring to–the one who lives upstairs. Because of the modifying function of relative clauses, they are also called “adjectival clauses.” They can be used to make clear which person or thing in the world we are talking about, as in the example just given, or they can be used to simply insert more information about a person or thing (“The singer, who really wasn’t a soprano, attempted to sing the high notes”; “The cheese, which had been sitting at the very back of the refrigerator, had gone bad”).