A sentence fragment is a piece of a sentence that's missing something essential, so it can't stand on its own as a complete thought. Think of it as a car that has an engine and wheels but no driver—it can't go anywhere by itself.
To be a complete sentence, a group of words must have 3 things:
A subject: The person, place, or thing doing the action.
A verb: The action the subject is doing.
A complete thought: It must make sense on its own.
Missing a subject: "Ran to the store to get milk."
Problem: Who ran? We don't know.
Correction: "She ran to the store to get milk."
Missing a verb: "The large, grey dog in the backyard."
Problem: What did the dog do? We don't know.
Correction: "The large, grey dog in the backyard barked loudly."
A dependent clause standing alone: "Because he was hungry."
Problem: The word "because" makes it a dependent clause. It needs to be attached to an independent clause to make sense.
Correction: "He ate a sandwich because he was hungry."
Fragments often appear in writing because the writer thinks the reader can guess or knows the missing information, but it's important to make sure every sentence is a complete and independent thought.