Martin, don't worry about thinking a question is stupid, just ask - if you don't know, you don't know.
You do indeed loosen the two screws / bolts on the underside of the frame. The whole eccentric then rotates (quite easily) and changes the position of the crank allowing you to adjust the chain tension.
You then tighten the screws back up when you are satisfied (don't over tension the chain - a bit of slack is desirable as is noted in many other posts on the forum) with the tension, making sure that the eccentric is aligned in the frame properly i.e. not sticking out either side.
The screws are pointed and make indentations in the softer material of the eccentric, these can be used again and again when you fit a new chain in the future and adjust at the same time. You can see the indents through the holes where the screws locate if the bike is upside down. In fact, if you can turn the bike upside down, i would undo one bolt and remove it completely and have a look. It'll help you understand what is going on.
Not explained very well technically, i'm sure others will contribute, but basically after you've done it a couple of times you'll find it a breeze!
Oh, and check the screws remain tight every now and again (with your fingers), they can come loose if, like me, you would rather not tighten them too much for fear of damaging the ebb!
Cheers!
Gary.
