Nominative is the grammar term for the form a word takes when it's the subject of a sentence, the one doing the action. In English, this matters mainly for pronouns: you say “I threw the ball” (not “me threw”), “he caught it” (not “him caught it”), and “they cheered” (not “them cheered”). The nominative forms are I, he, she, we, they, and who.

In languages like Latin, German, or Russian, the nominative case affects nouns, pronouns, and adjectives. Their forms actually change depending on whether they're doing the action or receiving it. English lost most of these changes centuries ago, which is why we can say “the dog chased the cat” or “the cat chased the dog” using the exact same word forms, relying on word order to show who's doing what.

When you diagram sentences and ask “who or what is doing the verb?” you're looking for the nominative. While English speakers rarely need to think about this consciously, understanding the nominative helps when learning other languages or analyzing how sentences work.