Axle load is the weight each axle transmits to the road. Every country sets its own limits for single, tandem and tridem axles and for drive axles; Directive 96/53 sets those for international traffic within the EU.
A truck can be below its maximum authorised mass and still overload one axle because the cargo is badly distributed. That is why the load planner looks at where the heavy pallets go, not only at how many there are.