"Normal wear and tear" is any type of damage or breakdown that is naturally expected from a machine. These types of damages are the responsibility of the equipment owner or rental company. If the service tech says that any issues are the result of normal wear and tear, then the machine will be serviced at no additional charge to you, the renter.
Wear and tear on a machine includes any issues that occur naturally over time and use of a machine. Worn down tracks, hoses cracking, small scrapes and dings from rocks, handles coming loose, lights dying out, etc are all examples of wear and tear.
Even though the equipment is a rental, daily checks should still take place. Look for anything that is loose, any lubricants or oils that are low, or if anything appears abnormal. Equipment should be serviced by rental companies before arriving on site but no one can predict when a wear and tear breakdown could happen. Circle checking equipment every day is the best way to identify wear and tear ahead of time.