In Premiere Pro, a clip points to a source file. Trimming a clip, or editing it in any way, does not affect the source file. 

For example, if you import a 30-minute file into Premiere Pro, you create a 30-minute clip that points to that source file and known as clip instance. If you trim this clip instance to a five-minute duration, the 30-minute source file remains on your hard disk, but the clip refers only to a five-minute section of it.

Source (master) clip

The clip originally imported into the Project panel. It is listed in the Project panel only once by default. If you delete a source clip from the Project panel, all of its instances are also deleted.

Clip instance

A dependent reference to a source clip, used in a sequence. Each time you add a clip to a sequence, you create another instance of the clip. A clip instance uses the name and source file reference used by its source clip. While clip instances are not listed in the Project panel, they are differentiated in the Source Monitor menu if you open instances there. The Source Monitor menu lists instances by name, sequence name, and in point.