Conventional lead acid batteries contain a liquid that is electrolyte.
Which is a mixture of sulfuric acid and water.
Under normal conditions a battery loses only water (in the form of vapor, hydrogen and oxygen), and only water should be replaced.
During operation, water goes through chemical reaction and breaks down into hydrogen and oxygen gas. These gases evaporate from battery and with time water level drops.
This way plates remain no longer covered with water. In order to keep the battery perform efficiently, it is important to top it up with distilled water.
Check the water level regularly and add distilled water only when necessary.