A case is a form of a word that mirrors how it's used.

There are three cases in English (I'm including a fourth that may be referred to as a pronoun).

Note: these can all be used in the plural form.

Subjective: This case is the word used for the subject. The singular forms are I, you, he, she, it, and they (gender neutral), and the plural forms are we, you, and they. Example of how they're used: He walked to the store. We drove to the cinema.

Objective: This case is the word used for the object. The singular forms are me, you, him, her, it, and them (gender neutral), and the plural forms are us, you, and them. Example of how they're used: The storm struck them. Nicolette went to the mall with me.

Possessive: This case is a word used for ownership. The singular forms are my, your, his, her, its, their (gender neutral), and the plural forms are our, your, and their. Example of how they're used: His bike crashed. Your mom just called.

Reflexive (sometimes called reflexive pronoun): This case is something that addresses an aforementioned being. The singular forms are myself, yourself, herself, himself, itself, and themselves (gender neutral), and the plural forms are ourselves, yourselves, and themselves. Example of how they're used: We regard ourselves as responsible adults. Try not to injure yourself.

There are other cases in different languages, so if you're planning on learning those, look it up or something, because I take French and Latin and haven't thoroughly divulged in their grammar yet.