To fill or charge your humidifier, you should remove it from the humidor and dampen the humidifier in a 50/50 solution of distilled water and propylene glycol (PG), usually referred to as activator solution. You are not looking to soak the humidifier, just get it damp, usually a few squirts of activator solution will be enough to keep it charged for a few months. This procedure should be repeated when your cigars start to get dry or when your hygrometer reads the humidity as being lower than you prefer.
Simply, PG is a hydroscopic substance. It absorbs moisture from the environment (like a salt shaker does in humid weather). The distilled water evaporates until the ambient humidity approaches 70%. At that point the PG won't allow any more moisture in the air. Conversely, if there is too much moisture in the air, the PG solution absorbs the excess, bringing the system down to 70% as well.
Some people figure that instead of buying distilled water, they will simply use tap water, or even filtered water. Filtered tap water is full of all the dissolved minerals which will ultimately plug your humidifier. The PH is balanced in tap water with chemicals which may interfere with the PG and cause variations in your humidity