What is a sentence fragment? [closed]

It is not a proper sentence, for the simple reason that it is missing a predicate. A good rule of thumb is that if the sentence doesn't have a verb, it probably does not have a predicate. The above sentence definitely has no verb.

In informal writing, this sentence would probably be fine, if the surrounding context provides an obvious predicate. For example, the context could be:

"All the superheroes were at the meeting place. All of them but Storm." (Apparently Storm is a superheroine.)

The first sentence above implies that the predicate is "were at the meeting place".

That means the second sentence actually is saying:

"All [the superheroes] but Storm were at the meeting place."