Probably some sort of gate valve to shut off water to the unit. The crusty white ring at the pipe fitting is mineral buildup that can happen in plumbing.

The mineral build up can either be from natural condensation that occurs when it’s cool, or the fitting might not be, or might not have been, 100% water tight. As long as you’re not seeing significant water on the floor, you’re fine.

It also looks like there is a little old white paint or sealant on the valve.