Agreements in sentences refer to the subjects agreeing with their verbs, and pronouns agreeing with their antecedents.
Subjects and their verbs
The subject and the main verb within a sentence must agree with each another in number, which is to say if a subject is singular, the verb must also be singular. Likewise, if a subject is plural, the verb must also be plural.
The subject and the main verb within a sentence must agree with each another in person as well. There are first person, second person, and third person.
Pronouns and their antecedents
A pronoun replaces an earlier noun which is called antecedent. The pronoun must agree with the antecedent it replaces in number (singular or plural), person (first, second, or third), and gender (masculine, feminine, or neuter).
Number: The pronoun must agree with the antecedent it replaces in number. This means a singular pronoun must replace a singular noun; and a plural pronoun must replace a plural noun.
A singular pronoun must replace a singular noun and a plural pronoun takes the place of plural nouns.