If your house has water pipes running through it as a form of heat, then you have a boiler. The boiler is usually located in the basement of the house. When the thermostat calls for more heat, the boiler will kick on and warm the water that flows through the heating pipes. There may be some loss of water over time that will eventually need to be added. How often water needs to be added will depend on the age and condition of both your home and your heating system.
Turn off the heat from the thermostat and allow the boiler to cool completely before filling. This is especially important if the sight glass is empty of any water.
Check the water level in the sight glass. The proper level for water is within one inch of the center.
Pull the handle of the water feed valve back to allow water to flow into the boiler. Keep a close eye on the water level in your sight glass. When the water has reached an appropriate level, push the handle of the feed valve up to the closed position.