The others are right, it means you're in solitary confinement. Most often you get put there for behavior problems. You are allowed to write letters and receive mail, but you lose your phone and visiting priviledges. And depending on the jail or prison, some allow you out for one hour a day in the "yard" for exercise (you're seperated from everyone else in a fenced in area) and some just allow you out either once a day or every other day to shower. It's not a fun place to be.