That little box is a small, resettable circuit breaker. At some point, too much current was drawn and it tripped. It could have been when hooking up batteries with an active load (fans, lights, refrigerator on), or if a wrench accidentally touched something it shouldn't have.
If it doesn't recur, no problem.
Sticks and Bricks: Raleigh, NC
2008 Cherokee 38P: at Ivor, VA permanently