If your vehicle uses lead-acid batteries - and most vehicles do - you need to be very careful that the level of liquid inside the battery doesn't fall too much.

Increasing the level of liquid in the battery is called watering, and is best done with the battery at full charge.
Charge the battery to its maximum extent and then detach the battery from the charger, and open the caps to each cell. Check the level of the liquid in each cell. You should know that this liquid can be quite corrosive, and that you should do this checking with something non-reactive, such as a strip of rubber.

If you find that the level of liquid has fallen to a considerable extent, you will need to top it up. This is done with distilled water. You cannot use anything but distilled water to top up the level of liquid in the cells. You must not use normal water, as it has a high mineral content, and this can cause serious problems with your battery or even damage it permanently.

Similarly, you cannot add sulfuric acid, as this would change the specific density of the electrolyte inside the cell. So, distilled water it is.

Add just enough distilled water so that the level of the liquid rises to about half an inch beneath the well of the vent of the cell.

Regular checks of your battery
If you're a conscientious owner and regularly check the levels of electrolyte in the battery, and top it up, you should make notes of just when you do this, and check how much time is usually required with your individual pattern of usage before you have to water your battery again.

This can be useful in preventing maintenance issues with the battery.

For example, if you know that you have to top up your battery every fifteen days or so, you will be less likely to miss that date. As a general rule, watering needs to be done more often when you are operating in a hot climate. Similarly, if you are putting the battery to heavy use, you will find that the electrolyte in the battery gets depleted that much faster.

Don't add more water than necessary
Please be careful not to add more distilled water than I have recommended and to not allow the level of liquid to rise to more than half an inch below the well of the vent. This is because the level of liquid inside the cell could rise during battery operation, and if it does, it could well leak out of the battery - and remember that the liquid is highly corrosive and could cause damage if it happens to leak.

While it is perfectly acceptable to add distilled water to a battery, spilling electrolyte is never a good idea, as this actually results in considerable dilution of the electrolyte if you then top it up with distilled water.

If you do this, you will find your battery's capacity severely reduced, so the best policy is really to prevent spills to the best of your ability.