A wheel clamp, also known as wheel boot, parking boot is a device made to prevent vehicles from being moved. We can use it for several reason like:

From one side for the wheel to be immobilized stopping the vehicle from moving for security reasons.
From the other side they protect the vehicle from being robbed as the wheel clamp will clearly show to thieves that the vehicle can not move.
And for long period caravan storing in campings.

Police use them as well when they want to make a fine to drivers.