Each vehicle is fitted with a radiator that needs equal parts water and coolant (antifreeze) to prevent the engine from overheating - do not use just water in the radiator. Coolant levels are easy to check because they are held in a see-through reservoir with a maximum and minimum line on the side.
Coolant always has a colour. If it is colourless, looks rusty, or has floaters; it is time to flush the system and ad new coolant.
If your coolant has an oily surface, immediately bring the vehicle to the mechanic.