An incomplete sentence, also referred to as a fragment, is any sentence without a subject and/or predicate.
I ran.
I'll go.
Run!
While all three of these sentences are short, they are complete sentences. Run! is a command, with the subject ('you') implied.
I --> subject
ran --> predicate, verb.
I --> subject
will go --> predicate, with helping verb (will) and main verb (go).
Fragmented sentences can be a very powerful tool in rhetoric and prose, if used sparingly.