Most of the time the coolant is low due to leaks in the coolant system, like radiator leak or water pump. blown head gasket, hoses and connections or a bad radiator cap can cause the coolant level to get low.
To avoid the costly repair, get your coolant system checked out by an experienced mechanic near you, a simple repair like a leaking hose will lead to overheating and it may cause engine damage in which it can be avoided.