What is “sentence variety”?
Sentence variety means that your sentences do not all follow the same pattern. For example, think of the sentence in books written for children just learning to read. Often, the sentences in these books all follow the same pattern of a subject followed by a verb:
The boy is happy. The girl is happy. The boy and girl are happy.
This repetition makes the books easy for new readers and boring for adults. In your writing, try varying the structure of your sentences. For more on sentence variety, see this page of the “Guide to Grammar and Writing” from Capital Community College.