Function Word
A word that does grammar work instead of carrying meaning: the, of, would, that. Half of everything you hear.
Function words are the grammatical machinery of a language: articles (the, a), prepositions (of, in, at), pronouns (it, they), conjunctions (and, but), and auxiliary verbs (would, can, do). English has only a few hundred, but they are used so constantly that they occupy nearly all the top spots on any frequency list, and about half the words in any sentence.
They are hard to learn from definitions because they do not mean things; they arrange things. The practical approach is learning them inside chunks ("a cup of coffee") until they become a feel rather than a fact.