Battery top up water is a type of water that is deionized or distilled to be used in replenishing the water and electrolyte solution in lead acid batteries.
Lead acid batteries are a mixture of water and sulfuric acid and when the water level drops in the lead acid battery due to evaporation or gas conversion during charging it can prevent the battery from holding a charge or become depleted quickly.
The electrolytes in a battery are a mixture of sulfuric acid and water and the water in the battery protects the batteries active material while the battery generates and produces power.
Without water in the battery, the active material within the battery will oxidize and the battery will then lose power.
To check if your battery needs water you can open the filler caps and ensure the electrolyte "fluid in the battery" covers the plates or the rectangular metal parts inside it fully.
If the car battery has filler caps that are removable then it's a type of battery that requires water as it's a maintenance based battery.
If the battery has no removable filler caps then it's a maintenance free battery that doesn't need water top up.
A marine or golf cart battery that is only used on the weekends may only require watering once a month.
A forklift that is used every day, may need to have its battery watered once a week.