They are circuit breakers for mains, they replaced fuses which used to be used.
As Saxo,said it's good practise to test the rcd. ( residual current device) when you connect up.
Press the red button and the switch should click down. Just click it back up afterwards.
------------- DS-There's more to life than football!!!
Pressing the RCD test button can be very useful as it will only trip if the supply is connected. Many modern vans etc don't seem to have any other indication that the supply is live.