Agreement in the English language
Agreement (also called concord) is the way a word (marked with an underline in this article) has the form appropriate to the person, number, or gender of the noun or pronoun (written here in bold). For example, the sentences "It exists." and "They exist." are grammatically correct, while "It exist." or "They exists." are grammatically incorrect.
Source: Wikipedia — Agreement in the English language (CC BY-SA 4.0)