A sentence fragment is a phrase that does not form a complete sentence; in other words, there is a component missing. This missing component might be the subject or the predicate, but, can also be when the sentence does not express a complete idea.
An example of a fragment: Changes need to be made.
The sentence above is a fragment since there is no subject (Where are the changes?). Fragments can be corrected by identifying the missing element and including it.
Revision: In this paper, changes need to be made.
An example of a fragment: In the story, written by Nicole, on December 27, 2016, in her house on the hill.
Notice here that although the sentence is quite long, it still contains no action (What about the story?).
Revision: In the story, written by Nicole, on December 27, 2016, in her house on the hill, readers are introduced to Nicholas.