Sarnath is a religious/archaeological site a few km out of Varanasi. It was originally a deer park, and was where Buddha preached his first sermon after achieving enlightenment. It’s one of the four pilgrimage sites of the life of Buddha, the others being the places of his birth (Lumbini in Nepal), his achievement of enlightenment (Bodh Gaya in Bihar, India) and his death (Kushinagar in Uttar Pradesh, India).
The place where a thing happened often isn’t very interesting, but in the case of Sarnath (and the other sites), the fact that many people believe it has value has made it interesting, because they’ve built lots of stuff there. It’s not currently a World Heritage site, but it can’t be long before it becomes one, so I thought I’d bag it pre-emptively.