An axle is the shaft that links the wheels on either side of a truck or trailer and bears a share of the vehicle’s total load. A tractor-trailer has several axles spread along its length to distribute weight across the road.
How weight sits across the axles matters a lot in trucking, because each axle and axle group has a legal limit. Spreading a load correctly — sometimes by adjusting sliding tandems — is what keeps a truck legal at the scales.