A sentence fragment may look and sound like a sentence, but it isn't one. It is a group of words that is missing either a subject or a verb or that doesn't express a complete thought.
To fix a fragment, add the missing part.
Fragment: Diving for the ball.
Complete sentence: Sara dove for the ball.
You can also fix some fragments by joining the fragment with a complete sentence.
Center field is one of the hardest defensive positions. Requiring long throws and much ground to cover.