There are two Bronze Age round barrows up here, but Aubrey Burl describes the one Neolithic barrow in his 'Prehistoric Avebury' (p128) and it's rather interesting.
A circular ditch was initially dug to mark the site out. A timber vault, open to the ENE sunrise was constructed, and in it laid a skeleton. The skeleton was already ancient, and had laid in the ground somewhere for more than a few decades (this was inferred from its brittleness). The man had had severe arthritis, and very worn teeth with a number of abcess cavities. Perhaps, Burl speculates, the man was a respected ancestor, who was brought from an old site to this new one, to imbue it with some kind of potency.
Later, the bones were strewn around the outside of the vault in (a series of?) rituals. The bones of the man's left foot remained inside the vault.
Later, earth and clay were mixed with a collection of broken pot pieces, charcoal, hundreds of flint flakes, a bone pin, pig bones, and oxen, sheep, deer, beaver and bird bones, and these formed a core to the chalk and rubble barrow which was then formed over the top in a kidney shape. Perhaps these things (that we might consider 'rubbish') were symbolic gifts: food and utensils which could be used in the afterlife? Maybe.