A measuring stick or ruler; also means a standard or measure used figuratively as a criterion for evaluation.
机の上に物差しがあるから、長さを測ってください。
There's a ruler on the desk; please measure the length.
成功の物差しは人によって違う。
The measure/standard of success differs from person to person.
製図用の物差しは精度が高い。
Rulers for drafting are highly precise.