Salt flats, also known as salt pans, are flat expanses of ground covered with salt and other minerals, usually shining white under the Sun. They are found in deserts, and are natural formations.
The largest and most popular salt flats in the world are Salar de Uyuni in Bolivia (the largest in the world), and Bonneville in Utah, USA (popular in the motorsports scene for its use as a high speed raceway).
You shouldn't tag a picture depicting a salt flat with Salar de Uyuni unless the artist and/or tags state that it is in fact Salar de Uyuni, since there are many other salt flats in the world.