Open hot just means that the wire that should be hot is not connected to the power supply. The neutral and ground are possibly still connected. When an outlet is labeled as "open hot," it means that the hot wire is not properly connected or is disconnected entirely. If there is an open hot, any GFCI upstream of the break in the circuit won't have power, and you won't be able to reset it (the button won't click and stay in). If you plug in the tester and no lights illuminate, that tells you the outlet is dead, and there's an open hot in the circuit somewhere.