More specifically, it's called a dependent clause or subordinate clause. These clauses cannot stand alone as sentences because they lack a complete thought. They often begin with a subordinating conjunction like "because," "although," "if," or "since."
Here's an example:
"Because she was tired."
This phrase contains a subject ("she") and a verb ("was") but doesn't express a complete thought. We need more information to understand what happened because of her tiredness.