Edit: Why is this down voted? It it literally the same thing. The firemen may call it a channel (and the sheet-metal is roughly of a shape that people in the metal industry would refer to as a "channel") but the terms are interchangeable, it's the lower outboard edge of the passenger cabin.
The obvious next question is: what’s a rocker panel?
Everyone here has an internet connection. Swapping an obscure term for an industry standard term should be enough to make search engines useful.
If I wanted to spoon feed people I'd go to Reddit.
One of the benefits of describing what a rocker panel is in your own words, or even quoting and providing a reference, is that it encourages discussions here.
From what I've been reading this morning the rocker panel is the structural and component of the vehicle immediately under the side doors, and also includes the visible panel.
Is that a good enough approximation? If you're able to provide more information that'd be great!