Subject verb agreement is when the person, thing, or group of people you are talking about agrees in number.
Example: They are running around the track. Since 'they' is plural, meaning more than one person, you use the verb 'are running' because 'are' is indicating a group.
Another example: She is running around the track. You use the verb 'is running' because 'she' is one girl, not many girls.
