Avebury is the site of the largest ancient monument in the British Isles. It was constructed during the Neolithic period in about 2850 BC.
A henge is a circular area enclosed by a bank or ditch, used for religious ceremonies in prehistoric times. 4-5,000 years ago there were as many as 200-300 henges in use. The most famous you’ve probably heard of is Stonehenge.
You can read more about the history of Avebury here.