I might be wrong but I see it more as 'power draw'.
If you turn off some lights in your house in doesn't make the remaining ones brighter, it simply draws less power.
Your power plant will convert fuel from your fuel tank into energy.
This energy supplies electricity to your systems (such as shields), electricity to your weapons, and plasma to your engines.
A higher class / larger power plant will produce more energy. This will allow you to run modules that need to draw more power.
A higher rated power plant will be more efficient and waste less energy / run cooler.
To get this energy from your power plant to your systems weapons and engines you need to distribute it among them.
The power distributor stores energy in capacitors, and also works to recharge these capacitors as the energy gets used. Better power distributors will have larger capacitors to satisfy the needs of high draw weapons like beam lasers, and will also recharge at a faster rate.
As a practical example, if you want something to keep on shooting for a long time... simply use a weapon with a low power draw (class 1 pulse laser) drawing from a really good power distributor (large capacitors and fast recharge rate) which runs off a really good power plant (producing an abundance of energy and less heat).
I hope that helps!!