What does side underrun protection do on trucks and trailers?

If you see bars down the side of a truck or trailer between the axles, this is side underrun protection and it's designed to stop cyclists, motorcyclists low vehicles and other road users falling, sliding or driving underneath the truck or trailer and being run over by the rear wheels.

Hino makes front under-run protection system standard equipment

NHTSA issues final rule on rear underride protection for semi

Semi-trailer truck - Wikipedia

Side underride guards: Life savers, or misdirected resources