Coarse salt is any salt ground or harvested into large, irregular crystals rather than fine grains. Because the crystals are bigger, they dissolve more slowly and provide a crunchy texture and bursts of saltiness rather than dissolving instantly into a dish.
Kosher salt is the most common coarse salt used in everyday cooking; despite its name, it isn't inherently more kosher than other salt - it's named for its traditional use in koshering meat, where the large crystals draw out blood efficiently. Rock salt is a coarser, less refined mined salt that often contains more impurities and is less commonly eaten directly; it's more often used as a bed for baking foods like oysters or clams, or in old-fashioned ice cream churns, than as a table or cooking seasoning. The two are related but not interchangeable.
Coarse salt (especially kosher salt) is widely used for seasoning meats and vegetables and for dry-brining and curing, where its slower dissolve helps season food evenly without over-salting the surface. Specialty finishing salts like fleur de sel and Himalayan pink salt are other coarse varieties, prized more for texture and appearance than for everyday seasoning.