Rock salt is a hardened version of common salt also known as halite, a name that comes for the Greek "halos" meaning "salt' and "lithos" meaning "rock." While found in a solid form the mineral is chemically the same as common sodium chloride, like that of table salt. Rock salt is commonly found in underground locations and along the shores of lakes and other bodies of salt water. The latter of these is a result of rock salts dissolving in the water, and being left on shore when the water evaporates.
Fill a pot that is safe to use on a stove or other heating source with warm water. Warm water requires less time to heat. The amount of water will depend on the amount of rock salt to be dissolved.
Heat the water on a heating source such as a stove top or other burner. Salt will dissolve in room temperature or cold water as well, but heating the water speeds up the process.
Place the rock salt into the warm water and allow it to settle. The rate of dissolution depends on the amount of salt and the temperature of the water.
Stir the water with a spoon or other instrument that can be used in hot water. If the rock salt reaches a point where it can no longer dissolve, then the water has been saturated with salt and become brine. If any salt remains, more water must be added, or it must be placed in a new pot of heated water to continue dissolution.
TL;DR (Too Long; Didn't Read)
All rock salt will dissolve in water. This method is the easiest way to do so quickly.