The battery is filled with sulfuric acid diluted with distilled water. Since water can evaporate, it is necessary to check the electrolyte level about once a month and, if necessary, top up with distilled water. The electrolyte level must be between the marks on the battery case (some models).
To top up with distilled water, remove the element covers and fill with water until the level is near the mark and it covers the elements. Replace the plastic covers.