Subject-verb agreement means that the ending of each verb in a sentence (or clause) matches its subject.
If the subject of the sentence is singular (he, she, it, or the name of one person or thing), the verb form must also be singular. In the present tense, that means the verb will end in ‘s.’ (A tree grows...)
If the subject is plural (more than one: we, they, several people or things)-- or if it is ‘you’ or ‘I’, the verb will not end in ‘s.’ (Trees grow...)
Subject-verb agreement is only a problem when the verb ending is different depending on 'person.' That means it is most often a problem in the 3rd-person singular present tense.
(The verb 'to be' is the only verb that changes form in the simple past tense. It uses 'was' for first & third person singular; 'were' for plurals and the second person.)
Using a verb ending that does not agree with the subject is a common grammar error. It's most likely when the sentence is complex or the subject is not obvious.
In the example above about the busy mother, the verb is singular. It doesn't matter that 'girls' is next to the verb. When you use prepositional phrases (of the girls, on the table, etc.) be careful! The noun in the prepositional phrase is never the subject. (See English Sentence Structure -- Phrases.)
It's still an important error to correct as you proofread your writing before sharing it. A lack of agreement can confuse the reader and will make the writer sound uneducated.
A related grammar point: check that the pronouns you use are correct for the nouns they
replace. See the paragraph below for an example.
“John and Mary were scientists in different labs. John sent Mary some data, and asked her to check his results. She wrote back to him, “Your results look good. We evaluated them against our earlier results, and they are quite similar.”
A one-question quiz
Who or what do “they” and “them” refer to?
A. Mary and John
D. the results
If you answered D, “the results,” you’re right!
Here's an excellent more detailed explanation from Purdue University.