A low loader, known in North America as a lowboy and sometimes known in the film industry as a process trailer, is a special kind of open trailer that rides exceptionally low to the ground.
Though not particular to the film industry, a low loader has an obvious use in film production. Vehicles or people can be placed onto a low loader, whilst cameras are mounted closer to the cab, and lighting fixtures are secured around the subject being filmed. Depending on the angle chosen, it can then appear that the the person or vehicle is travelling at the speed of lorry (or truck) to which the low loader is attached. Hence, it can seem that actors are driving a car, riding a horse, or even potentially running or flying, when in fact they are actually securely attached to a trailer.
Low loaders, in one form another, have been used since the earliest days of filmmaking, but weren't particularly used in Doctor Who filming until the 1996 telemovie. Since then, they've been used by the BBC Wales production team on occasion. Doctor Who Confidential featured a segment on the use of low loaders in Nemesis.