I like how the return comes up through the bottom of the tank and doesn't hang on the back.
Like everyone is saying,Add water.
Turn on your pump and keep it on. It will make slurping noises until you add water.
Add water until it comes up to a desired level in your sump. Add water in the sump with pump running and you will see it drop some as it pumps it up to the tank (The water level in the aquarium should rise some). As it rises more water volume will come down your overflow and start to even out with your pump/water level, keep adding until it levels of and stays consistent in the height you want in the pump chamber.
I don't know if this is a new set up or has been running a while. Keep an eye on your tank water level. Make sure you don't have a clogged water return.
What is happening is water is evaporating and the level in your aquarium is dropping. When this happens you have less water flowing into the overflow chamber down to your sump and the pump is pumping the water faster than the overflow, lowering your sump water level.
I have to add about a quart of water a day from evaporation. My pump chamber drops about an inch a day so I have to add water every day. (I should add a auto top of system.)
I also don't see a ball valve on your Pump return. It's nice to have a ball valve for fine tuning your pump. You might have one, but I don't see it in this pic.