Sorry to be such a rookie, but here goes -

What am I heating up (or whatever I am doing) when I plug my truck in?

Thanks.

 

You are heating the coolant with the block heater element.

 

So, it's ultimately an engine heater?

 

In simplest terms, yes. You're heating the coolant in the engine block, which keeps the block warm.

 

In cold climates, motor oil turns to the thickness of molasses or even thicker. The oil pump needs to push this thru cold metal at start up and retards the dilution of the bearings etc with oil, extending the time it takes to loosen things up and get to operating temps. A warm block will effectively thin the oil to get it where it needs to be faster and hopefully reduce unnecessary wear on moving parts. In the polar regions they even use heat blankets and engine shrouds to keep things moving during idling. It can get that cold.
Diesels don't produce much by product heat when idling and not under load and they like to run a bit hot for max efficiency. If your temps are below freezing and you are not using synthetics I would suggest plugging it in. I use a timer so it only runs for about 3 hours before I head out. This saves electricity becasue the block heater pulls a lot.
I hope you find this helpful.
Kurt