The counterpart of a dependent clause. A dependent clause has a subject and verb, but it cannot stand on its own. An independent clause is simply a simple sentence. It can stand on its own.
For example, the following clause is independent:
Jim drove his car over the bridge.
When a DMW is added to it, it becomes a dependent clause.
When Jim drove his car over the bridge,
The word “when” is a DMW and makes this clause dependent. It is called dependent because it must be attached to an independent clause in order for it to function as a complete thought.