A "Block" is a stretch of track, that is protected at the rear end, and the front, of a train, where the train could sit and wait at a red signal, resting assured that a rear end, or head end, collision would not occur.
The preceding block distant signal, behind the rear end of the train, would show a "Caution" signal, that a train ahead is protected in the block ahead, and the oncoming train should be aware of the stopped train, prepared to stop at a reduced/slow speed.