The axle is a bar or shaft used in turning the wheels. The transmission helps turn the axle, that turns the wheels. There is sometimes a front axle and a back axle that usually has a differential in the middle that joins the front pair of tires and back pair so they move at the same time. Since an axle is a solid metal, it is difficult to break it.
If more weight is added, the turning capacity of the axle will drop down. The worst enemy of an axle is rust. It gradually leads to faulty performance sometimes it also causes a breakdown. Cleaning the undercarriage, if salt is used on the road, and driving carefully on rough roads will protect your axle.