Sliding tandems are the trailer’s rear axle group mounted on rails so they can be repositioned forward or backward. Moving them changes how the load’s weight is shared between the trailer axles and the tractor’s drive axles.
This adjustment is a driver’s main tool for getting legal at the scales: if one axle group is over its axle rating, sliding the tandems can rebalance the weight without touching the freight. It’s a routine step before a weigh station when a load sits unevenly.