Subject verb agreement is when your subject and you verb basically agree. If your subject is singular than your verb also has to be singular.
Example.
DOG= noun subject CHASES=verb
The DOG CHASES the cat.
In the sentence you are talking about a single dog so it is singular. Because the noun subject is singular the verb then has to be singular. It wouldn't sound right if you had a plural verb with a singular subject.
Example.
DOGS= subject CHASE= verb
The DOGS CHASE the cat.
Now in this sentence they are talking about more than one dog so it becomes plural. Because the subject is now plural the verb also has to become plural.
