What does "Evaporative Emission Control System Malfunction" mean? How difficult is this to fix?

When you pump gas in to your tank it displaces air and fumes.. That air is pushed by the displacement in to an activated-charcoal canister that is then purged by intake vacuum when you start the engine. Called EVAP. It's part of emissions so not like the old days when there was no rubber boot on the gas pump nozzle and the fumes came right out in your face and in to the air we all breathe. It depends on what is wrong with your EVAP to make a cost guess

Typical system..click to enlarge

My gas light had been on 3 day I full up tank see if it would go off but it didn't can u tell me what problem is