A battery is properly called an electrical cell, because a battery is the word for a line of more than one cell. Each cell contains a number of chemicals which work to change chemical energy into electrical energy.
Many cells can only be used once. They start to run down as the chemicals are used up. Some cells can be recharged, just like a car battery (which has 6 cells in it to make 12 volts). In a rechargeable cell, putting electricity into the cell changes the chemicals back to how they started, so that when the cell is fully charged, the chemicals can make electricity again.