It’s a remarkable story. Pitt-Rivers (not his original name but taken on via marriage) unexpectedly inherited a fortune so started spending it on his interests. These included history - he started digging up historic sites on his properties, then realised this had to be done systematically. This was the foundation of archeology. The museum contains the artefacts he found as well as those he bought from collectors all around the world.
It is really overwhelming - the museum is literally packed with exhibits. There is just too much to see and take in on one visit.