Tether is supposed to able to be traded in for a dollar. It closely resembles the value of a dollar. In a pinch, at an exchange that doesn't have USD liquidation, tether appears to hold its value when the market is getting killed. However, when people go to turn their tether in for dollars, if there was a mass claim, they may not be there to handle it. I hear they were hacked as well. All I know :)