Playa lakes are usually inland lakes in desert regions. All rocks contain salt, and when rain falls, the rock is partly dissolved, so salt goes into the water. In most lakes the water then flows out via rivers to the sea. But playa lakes have no exit, and so the water goes no farther. In a desert, the hot sunshine simply evaporates it, so that the amount of salt in the lake gets higher and higher. Eventually a point comes when the water cannot hold all of the salt in solution, and some of it comes out to make salt crusts, like the one in California you can see in this picture.

Playa lakes are often mined for their salt. One of the biggest playa lakes is the Dead Sea in the Middle East. That lake is so salty you cannot sink in the water.