Modifiers are words that describe or qualify other words. Generally speaking, all adjectives and adverbs are called modifiers in contemporary English. Even adjective phrases and clauses, as well as adverb phrases and clauses, fall within the scope of this concept. The italicised words (or expressions) in the following sentence will suffice to illustrate: Those boys who came here today are surely the children of that handsome man in your garden.
[Those = determiner (demonstrative adjective); who came here today = adjective clause; surely = adverb; the = determiner (definite article); that = determiner (demonstrative adjective); handsome = adjective (descriptive/attributive); in your garden = adjective phrase. ]
From the above, it is evident that some modifiers are single words while others are word clusters.