The pump will run dry for a set amount of time (a few minutes), it will eventually shut off once the magic eye thing in your sump/tank registers that the water level is not rising. Once the pump shuts off due to lack of water, it will not attempt to run again, the light on the controller box will start flashing to let you know there is an issue that needs to be taken care of.
This will also happen if there is an air bubble in the osmolator pump, the pump is very weak and sometimes it cannot prime itself.
It is not a good idea to let the pump run dry, even with the set timeout there is still the possibility of permanent damage to the pump.
There is also a shut off for if the pump runs too long, I think the time out is around 10 minutes.
I have Osmolators on two of my tanks and I love them.