What does the engine warning light coming on mean in a ford ka?

I had to get a sensor replaced on my Ford ka which was causing the engine management light (which I thought was the petrol tank light to my eternal shame) to go on. When it was replaced it stayed on for about 3 or 4 days and then went back to normal. Read up on it before you go to a mechanic as if you don't know anything about it they will charge you more, the bastards