Coolant regulates engine temperature, prevents freezing, overheating and corrosion.
It is not just for winter. Normal coolant is replaced every 2 years or 40,000 km. Long-life coolant can last 5 years or 100,000 km.
If the level is low, add the same color and type of coolant. In emergencies, you may add a small amount of distilled water, but not tap water, which causes scale and corrosion.
Different colors should not be mixed. Old coolant can cause overheating and freezing damage.