if you are talkin about the button which is where you put the ignition key it is there to allow you to put on the parking lights while the car is without the ignition....and u can remove the key as well....
To do this all you have to do is put in the key....press the button and while holding it depressed you have to turn the key anti-clockwise direction
Yeah I only found out about this in the past week or two -- think I spotted it in the Porter manual, or maybe it was just reading through the Uno user manual . The button in question is the kidney-shaped button on the side of the ignition keyhole (not on the key itself) -- see the attached pic.
As Drinu says, with the key in the ignition, turn it towards you (i.e. the opposite direction to starting the car) while pushing the button in. The parking lights then come on and you can take out the key while leaving the lights on.