It means how to place a verb in a sentence.In sentence the main verb must be related to its subject. There is hard and fast rules. But one has to follow the bellow rules for placing the main verb in a sentence.
1.In every sentence,the main verb must be used according to the number and person of its subject that the number and person of a verb shall be as like as its subject.
If the subject is third person and singular,the verb will be third person and singular number. If the subject is first person and plural number ,the verb will be first person and plural number.For example
The colour of his eyes is blue.
The colours of his dress are white and blue.