How must a verb agree with its subject?
Very simply, the answer to this question relates to the number of things that are being refered to in the sentence. Subject verb agreement therefore means that the number of the subject, or the object, corresponds with the form of the verb. Therefore, see if you can spot what the problem is with the following example:
The dogs runs through the park.
Did you spot it? The subject of the sentence, which is the thing or things doing the action, is plural, because there is more than one dog. The verb however does not agree with the subject as it is third person singular, when it should be third person plural: "run." The phrase subject verb agreement therefore relates to the way in which we have to balance the verb with the subject, especially thinking about number.