A benchmark is a standard that other similar items can be compared to — that way everything is measured against a common standard.
Benchmark originally referred to a mark made on a wall or building by a surveyor for use as a reference point. The word still retains that meaning, but nowadays you’re more likely to hear it used in a figurative sense to describe a standard used to measure the worth of something. A particular item is selected as a benchmark, and other similar items are compared to the benchmark. A popular sitcom may be the benchmark other shows are judged against.