First we have to understand what a battery is made from. A battery consists of a number of separate cells. Each cell contains two different plates. The plates are held fixed and separated from each other. They are surrounded ("wetted" or "soaked") by a special chemical fluid or paste which conducts electricity.
One plate is called the "positive" ("+ve"), the other is called the "negative". ("-ve"). Every cell works because the plates react with the special chemical in such a way that an electrical current can be drawn off from the two terminals of the cell.
As current is drawn from the cell its plates - and the special chemical in which the plates are immersed - all gradually change their chemical composition. The result is that, as current is drawn off, the charge stored in the cell is reduced.
Another result is that the voltage given out by the cell drops. If nothing is done to replace the charge that has been taken from the cell then, eventually, a point is reached when no more charge can be taken out. That happens because the chemical composition of the plates and the special fluid can change no further. The cell is then called "flat" or "dead".