That is a manual resetting circuit breaker. It is needed to protect the wiring between the battery and the 12 volt DC power distribution panel. http://www1.cooperbussmann.com/pdf/0da13f4c-9da6-4f03-aa99-0ec06ec82d7a.pdf
If it is bad it can be replaced with a standard automotive type circuit breaker available at most automotive stores. Some are auto-resetting and some require you to push the button after it has tripped.
You may need to change the type of terminals on the wires as most automotive style circuit breakers have posts.