what is agreement

Agreement means that the verb changes depending on the type of noun it refers to.
Examples:

‘I like him.’ ‘I’ is singular and a first person pronoun, so the verb is ‘like’.
‘She likes him.’ ‘She’ is singular, but a third person pronoun (so are ‘it’ and ‘he’), so the verb is ‘likes’.
‘This student likes grammar.’ ‘Student’ is a singular noun, so the verb is ‘likes’.
That means "verb" should agree with "subject" . We use different type of "verb" (with "s" or no "s") to be consistent with different subject (first / third person pronoun)