A salt pan is a place where water pools when it rains. A salt pan would be a lake or a pond if it was in a location in a climate where the rate of water evaporation wasn't faster than the rate of water precipitation, i.e., if it wasn't in a desert. If the water is unable to drain into the ground, it sits on the surface until it evaporates. When the water evaporates it leaves behind whatever minerals were dissolved in it. Over thousands of years, the minerals (usually salts) accumulate until the the surface is white with it. There are many salt pans in the south-western United States. You can see them from an airliner; from that height a salt pan can look like a frozen lake or a snowfield.
Salt pans can be dangerous. You should never attempt to cross a salt pan on foot or in a vehicle unless you know exactly what you are doing. The crust of salt can conceal a quagmire of mud that can engulf a truck. One section of the eastern Sahara desert contains many such raps; it was a strategic barrier during World War II.
The two best-known salt pans in the USA are probably the one where Burning Man Festival is held, and the Salt Lake Desert, a vast flat plain where many land speed records have been set.
The Etosha Pan in the Etosha National Park in Namibia is another prominent example of a salt pan.