A device in the water line to the black tank. It is to prevent any chance of the black tank contents being sucked back into the water lines. If the water supply should fail and attempt to siphon water backwards in the system, the "vacuum breaker" would admit air and break the siphon preventing contamination of the water supply. Sometimes the internal valve sticks or fails and water leaks out when the black tank flush is used. It can be a little water or a lot.
Like RedHorse1 said, it is above floor level and usually a cabinet. Ours is under our bathroom sink, but that location may not be helpful to you.
Sherm & Terry w/rescues Steve (std Poodle) & Eydie (min Schnauzer)
2015 Jay Flight 27RLS (Camped: 102 nights 2015, 90 nights 2016)
2006 Ford F350 6.0 PSD, Lariat, 4WD, CC, LB, SRW, auto., Camper pkg.