i did say i had a few questions to ask

i have an intermitant engine warning light come on and it says on my consul that anit pollution is the fault.
i can then turn off my engine and it goes off. so my local garage says they cant do owt unless it is showing at the time
can some one explain to me what this anti pollution thing is and is it easy rectified?
oh it only started after i changed a headlight bulb a few weeks ago is this a coincidence
thanks in advance