Dosing is simply the action of adding elements to your aquarium that otherwise would not have been available to your coral with water changes alone. The three main elements tested are alkalinity (KH), calcium (Ca) and magnesium (Mg).
Most new hobbyists will begin dosing by testing for these three parameters and realising they are becoming or have become depleted. If they had done their research, the depletion was to be expected.
The next step is to work out what the take-up rate, or depletion rate, is per day and calculate how much to dose to replenish the loss.
Manual dosing means measuring out your elements and adding them daily by hand, but it has its drawbacks. It requires that you are always there to dose, and that you don’t forget to dose. As your reef tank grows it becomes more demanding, which means elements are utilised quicker. If you are adding elements once a day you are potentially creating unwanted fluctuations in levels.
This is a good way to start, but eventually the progression is normally towards automatic dosing.