A sentence fragment is a word, a phrase, or a dependent clause used (and punctuated) as a sentence.
during lunch (PHRASE)
During lunch. (PHRASE PUNCTUATED AS SENTENCE)
I’m going to call her during lunch. (CORRECT)
I’m going to call her. During lunch. (INCORRECT)
when the sun comes up (DEPENDENT CLAUSE)
When the sun comes up. (DEPENDENT CLAUSE PUNCTUATED AS SENTENCE)
The rooster crows when the sun comes up. (CORRECT)
When the sun comes up, the rooster crows. (CORRECT)
The rooster crows. When the sun comes up. (INCORRECT)
A dependent clause cannot stand alone as a complete sentence.
AND SEE: Sentence fragments: pro and con
*NOTE: There are several different kinds of phrases.