When adding a driver, you'll be asked to specify the driver's vehicle capacity. It might be confusing at first why there aren't any units - KG, boxes, etc.
Capacity is an arbitrary number that you can decide for your drivers - as a florist, that could be 10 bouquets; as a baker, that could be 30 boxes of cupcakes; as a furniture mover, that could be 100kg worth of furniture.
When you add a job, the load that you specify represents the amount of vehicle capacity the items in that job take up. If your driver's vehicle capacity is 30 boxes of cupcakes, and the job's load is 5, this means that you have 25 boxes of capacity remaining for that driver's vehicle.
This capacity and job loads are taken into account when Routo automatically plans your routes.