Hi, there,
The answer is a dependent clause, also known as a subordinate clause. Some examples include sentences that begin with "While" or "Because". Some dependent clauses begin with subordinating conjunctions or relative pronouns, such as, for example: "While I was waiting for the bus, it started to rain." Here "while" is the subordinating conjunction, and there is a comma required between the dependent clause and the following independent clause. If reversed, "It started to rain while I was waiting for the bus" does not require a comma.
I hope this answered your question! Best of luck with your studies! :-)