Charcoal canister what is that?

I read about a Charcoal canister. What is it and how does it work?

Put simply, petrol is a gas not a liquid so the CC stores fumes from the fuel tank so as not to pollute the atmosphere, then releases the stored gas into the engine combustion chamber to be burnt together with the normal air/fuel charge but not at idle as this could stall the engine. Hope this helps.

How can I see/test if it is working ok? I have over pressure when releasing the fuel cap. Has this perhaps something to do with the canister?

What do you mean over-pressure? Relative to what? There should be pressure in there, it helps keep the fumes in and water out! If there is over-pressure, the fuel filler cap will vent it. I think you are concerned about something that is normal. You will notice it more when the tank is less full or when you've left the car parked in the heat. If you can smell petrol vapour when the car has been parked for a while, then your CC might not be working, but otherwise don't worry.

If I release the fuel filler cap with an almost empty tank. The over pressure escapes, so then You smell a bit of Petrol... But that is normal?