Subject verb agreement is an exciting topic. Basically, if a subject is singular, the verb must be singular. If a subject is plural, the verb must be plural. A firm grasp on the rules of subject verb agreement is essential. In fact, we outline 20 of those rules here.
When subjects and verbs don't agree, it can confuse the listener or reader. This will distract them from whatever information you're trying to impart upon them. So, instead of enjoying your prose, they'll wonder if you got your verb tenses wrong or, perhaps, selected the wrong subject. Let's break this down a little further.