In order to bring two like charges near each other work must be done. In order to separate two opposite charges, work must be done. Remember that whenever work gets done, energy changes form.
As the monkey does work on the positive charge, he increases the energy of that charge. The closer he brings it, the more electrical potential energy it has. When he releases the charge, work gets done on the charge which changes its energy from electrical potential energy to kinetic energy.
Every time he brings the charge back, he does work on the charge. If he brought the charge closer to the other object, it would have more electrical potential energy. If he brought 2 or 3 charges instead of one, then he would have had to do more work so he would have created more electrical potential energy. Electrical potential energy could be measured in Joules just like any other form of energy.
Since the electrical potential energy can change depending on the amount of charge you are moving, it is helpful to describe the electrical potential energy per unit of charge. This is known as electrical potential (NOTE: this sounds very similar to electrical potential energy, but it is not!)
As a formula it is written like this: The energy per unit of charge is often called voltage so it is symbolized with the capital letter V. Work or energy can be measured in Joules and charge is measured in Coulombs so the electrical potential can be measured in Joules per Coulomb which has been defined as a volt.
For things like batteries, we specify the potential difference of the charges within the battery. So a “D-cell” has a rating of 1.5 volts which means that for every Coulomb of charge that moves from the negative side of the cell to the positive side will do 1.5 Joules worth of work. A “AA-cell” also has a rating of 1.5 volts so each Coulomb of charge that moves from one side to the other can and will do 1.5 Joules worth of work. The difference between the D-cell and the AA-cell is that the D-cell has more Coulombs worth of charge, so it will last longer. The AA-cell may only light a light bulb for 15 minutes while the D-cell may keep the same bulb lit for several hours. As a result of having more charge, the D-cell has more energy and can do more work, but it will still do work at the same rate (or has the same power) as the AA-cell. Notice in the animation that every charge that passes through the light bulb does 1.5 joules worth of work which makes it heat up and give off light. Contrast this with a wall outlet that has a potential difference of a 12 volt car battery. In the 12 volt car battery, every coulomb of charge that moves from one side to the other does 12 Joules worth of work. In a 120 volt electrical outlet, every Coulomb of charge does 120 Joules worth of work as it moves from one side of the outlet to the other.