Agreement means conformity.  So, the subject verb agreement is the conformity between the subject and the verb (verb).  Because verbs in English are not like Indonesian which are almost always the same shape regardless of the subject, a verb must be adapted to the subject of the sentence.
 Example:
The kidplays kite.
Kid: anak kecil (singular)
Plays: bermain (singular verb)
The kidsplay kite.
Kids: anak-anak (plural)
Play: bermain (plural verb)