Normal wear and tear is the expected decline in the condition of a property due to normal everyday use. It is deterioration that occurs in the course of living in a property and not caused by abuse or neglect or related to items a tenant brings into the home (installing shelving or picture hanging etc).
Using carpet as an example:
Normal wear and tear - carpet worn down at the entry from day to day foot traffic
Deduction- carpet contains rips, tears, stains or smells from abuse or neglect