I'm installing 2 wall mount light fixtures, the first one was easy it was just one for one with black,white, and ground. The second box in the wall though is pre-wired with 2 black, 2 white, and 2 ground. My question is, do I connect all 3 of each together. Attached is a picture of the wiring coming out of the box.
It appears likely that one cable, with a black, white and ground, is the feed (source of power), and the other is a pass through to the next fixture or outlet.
In that case, you would simply join your fixtures wires to the existing bundles, matching colors.
This assumes that switching is already in place, or the fixture has internal switches.