A grammar label, not an everyday English word. In some languages a noun (or pronoun) changes its shape to show its job in the sentence. The dative is the shape you use for the person or thing you give something to, or do something for — the one who receives, not the thing that is given. In “I gave my sister a book,” my sister is that person. English mostly shows this with word order and little words like to and for, not with a special ending. German, Latin, Russian, and Greek still have a real dative form; Taiwan students often meet it first in German class (dem Mann, der Frau). People also say dative as an adjective: the dative case, a dative ending. Though actually, English used to have dative endings too, centuries ago, and a few old traces remain in pronouns (give me the book). Some school books still stamp “dative” on English to/for phrases, but that is a Latin name laid over a language that does not really mark it that way any more. You need this word when you talk about other languages, not when you buy bread.