Brine tank
The salt-holding tank that lets a water softener recharge itself.
The brine tank is the shorter of a water softener's two tanks — the one you pour salt into. Water sits in the bottom and dissolves the salt into a strong brine; during regeneration the softener draws that brine through the resin to recharge it. Keep it at least half full, watch for a salt bridge (a hard crust with an air gap) or salt mushing (sludge at the bottom), and clean the tank out once a year.