I'll give you an analogy I used to give my students...I’m not up on my terminology. I put my test light to a chassis ground and touched the red wire coming from the rec/reg and it lit up with the key off. I just want to make sure that’s all good.
The faucet in your sink has 'potential' energy. There's about 15-20psi of water pressure there. The same thing's in your wires... potential energy in the form of 12V. Just like you can put a pressure gauge on the faucet, you can do the same with your wire... in the form of a 'voltmeter,' or in your case, a test light. You can see the potential energy in both.
When you open the tap, you're releasing that energy in the form of flow. You can measure that as GPM or gallons per minute of flow. When we flip the switch on your wire, we're doing the same thing... releasing energy... in the form of 'current'... or electrical 'flow.' It's measured in amps.
We'll save 'power' for another day...