The process works as follows: a boulder sitting on the surface at mid-latitudes casts a shadow in winter. The continually shadowed area behind the boulder is very cold, so cold that water ice accumulates in winter.
When the Sun rises again in spring, the ice suddenly heats up. In detailed model calculations, the temperature rises from -128 degrees Celsius in the morning to -10 degrees Celsius at noon, a huge change over a quarter of a day. Over such a short time, not all of the frost is lost to the atmosphere.
Salt depresses the melting point of H2O, so on salt-rich ground, water ice will melt at -10 degrees Celsius. Brines, or salty water, will form until all of the ice has either turned to liquid or vapor. Next Mars year, the same process repeats.
The shadowed areas behind the boulders are so cold in winter that not only water frost but also carbon dioxide ice builds up. For Mars, the first day without carbon dioxide ice in spring is called the "crocus date." Melting occurs on or immediately after the crocus date, and therefore the term "crocus melting".