The sentence fragment is a clause masquerading as a complete sentence. It may contain a verb (a doing word) but the verb does not set the action in time, i.e. it does not tell us when or if an action was, is being, will be or might be performed. Such verbs are called 'driving verbs', 'main verbs' or 'finite verbs'. These terms - driving, main or finite - are interchangeable. To be a complete, a sentence needs a main verb.
Examples of fragments:
'Reading’ is not a main verb. It is a style of verb called a gerund, i.e. a verb working as a noun. It does not power the sentence by telling us when the action was, is, will, or could be performed.
‘Can’ is a (modal) verb but the sentence leaves the reader hanging in mid air because it lacks a main clause telling us what must be done 'so that you can win ...'
I blame political speech-writers. They love fragments. In Tony Blair's 1998 oration to the Labour party, there were over a hundred fragments. He is often laughed at for this, but fragments have their uses: they make perfect soundbites.
For certain types of writing, including novels and journalism, fragments are very useful. In academic essays, however, sentences need to be complete.
Here is an easy way of identifying fragments: