A block composed primarily of common
salt (
sodium chloride), perhaps some
calcium, and often containing small amounts of other, trace
minerals such as
copper,
magnesium,
zinc,
iron, and
cobalt.
Salt licks are placed where livestock can easily access them, and provide crucial nutrients for the animals. Like humans they usually don't need all the sodium, which serves as a sort of substrate for the scarcer elements. The desire for salt appears to be intrinsic to the metabolisms of most mammals; wild deer will often leap fences to taste farmers' salt licks when they can get away with it.
As animals often cluster around blocks of salt, the idiom has been extended to humans as well. A "salt lick" can be any sort of popular gathering place.