In grammar, person is how we distinguish between the speaker or writer (first person), the person being addressed (second person), and any other people, objects, ideas, etc. referred to (third person).
Person is expressed through the different personal pronouns, such as “I” (first-person pronoun), “you” (second-person pronoun), and “they” (third-person pronoun). It also affects how verbs are conjugated, due to subject-verb agreement (e.g., “I am” vs. “you are”).
In fiction, a first-person narrative is one written directly from the perspective of the protagonist. A third-person narrative describes the protagonist from the perspective of a separate narrator. A second-person narrative (very rare) addresses the reader as if they were the protagonist.