Qualify in grammar
In the study of grammar, the verb 'to qualify' has a specific and limited technical meaning. It is used to mean the effect an adjective has on a noun. Given that nouns are the names of things, and adjectives 'describe' them, the verb qualify is chosen as a word that accounts precisely for the relationship between an adjective and a noun. The root meaning of 'to qualify' is "to invest with a quality or qualities" (OED). From thjis comes the special meaning in grammar, "b. Gram. Of an adj.: To express some quality belonging to (a noun)."
In traditional grammar as taught in UK schools in the first half of the twentieth century and beyond, the distinction between adjectives qualifying nouns and adverbs modifying verbs was held to be of great importance. As OED gives one (subordinate) meaning of qualify as "Of an adv.: To modify", I cannot now find it of huge significance - unless you are writing for a pedant.