Je mange une pomme.
English: I eat an apple.
In French, 'subject-verb agreement' means that the verb changes to match the subject in person (who is doing the action) and number (how many people).
You use subject-verb agreement every time you make a sentence in French. The verb form must match the subject (who is doing the action) in both number (singular/plural) and person (I, you, he/she, we, you all, they).
Je mange une pomme.
English: I eat an apple.
Tu regardes la télévision.
English: You watch television.
Il chante.
English: He sings.
Nous aimons le chocolat.
English: We like chocolate.
Elles jouent dans le parc.
English: They (feminine) play in the park.