At a basic level, there are things we call power sources because they have a property that makes them want to make electrons move. In batteries, this is because of the metals involved; in generators, moving magnets induce current in wires.
As an analogy, some water sources are natural because of water flowing down from a mountain, and some are pumped up from under ground.
Some power sources are more forceful than others, just like some water sources have higher pressures than others.
This is what voltage is: the pressure that something has to force electrons to move.
If these electrons are met with something that has little resistance, electrons will flow quickly - just like if a hose is left just running, water will run quickly. If there's more resistance, electrons will flow more slowly - just like pushing water through a small "eco-friendly" nozzle (less water will flow because there's more resistance). Regardless of what it's connected to, the pressure is still the same from the source.
Another thing to mention is that power sources not only have a voltage (pressure), but they also have a maximum current/"amperage" (flow rate).
Transformers trade off one voltage/current combination (e.g. 120V @ 10A) for another (e.g. 12V @ 100A), where the total "power" (Watts, which is volts times amps) is the same.
There's a relatively complicated reason why we sometimes measure this in VA ("volts times amps") instead of W ("watts", which is volts times amps).
So, if you have a 100 kVA transformer (which is 100,000 VA), you could run 1 Amp at 100,000 Volts on one side and have it convert it to 100 Amps at 1,000 Volts (or 1kV).
Incidentally, water also has a "transformer". If you have water flowing in a hose at 1 ft/s @ 10 PSI and you constrict the hose to 1/2 its size, the water will be flowing 2 ft/s @ 5 PSI. The pressure dropped and the velocity went up, but the total amount of water flowing is the same.