A pronoun is a word that is used instead of a noun (e.g. I, you, he, she, it, this, that, they, them). Pronouns referring to a person can be linked to someone's gender identity. It's important to ask someone their pronouns rather than guessing based on what they look like. Or use gender-neutral ones (they/them).
See: Gender Identity, Gender Expression, Gender Attribution and Misgender.