What's a sentence fragment?
A sentence fragment is an incomplete sentence.
The most frequent type of sentence fragment error occurs when a writer attempts to use a subordinate clause as a sentence. Remember that a subordinate clause needs to be connected to a main clause.
Example
Because Edwin Hubble’s amazingly productive research lead to so many discoveries and changed the way we think about the universe.
This is a sentence fragment because it is only an adverbial clause. The reader does not know what results from this group of words. What happens "because Edwin Hubble's amazingly productive research lead to so many discoveries..."? The main clause would answer this question.
When sentence fragments are in the context of a longer passage, they can be more difficult to detect than when they are considered in isolation. Therefore, a good trick for catching your own errors is to consider each sentence in your writing as a separate entity. Each sentence must stand as complete on its own; it must not depend on others around it to sound finished.