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There is much debate as to the purpose of the archaeological feature known as a "Burnt Mound". They date from the late Neolithic all through the Bronze Age (c.3200–900 BC) with a few examples from the subsequent Iron Age - though the dates and extent of these 'Ages' vary considerably by region and by historical authority. Effectively they can be thought of as a Bronze Age phenomenon.
There are two schools of thought as to how quickly they were created - rapidly, as the site of a temporary camp, or very slowly over decades or even centuries.
As to their purpose - suggestions have included cooking, bathing, dyeing, tanning and various other ideas. What is clear is that they were not burial sites, as they never contain human bones or ashes, but do always contain charcoal and often animal bones.
They are common in the north of Scotland with many in the Orkney and Shetland Islands. They are also plentiful in Ireland.
They are quite complicated structures usually featuring some kind of hearth and a trough all surrounded by a mound of shattered rock mixed with charcoal and sometimes animal bones. Some also contain a cist (a stone lined box, most usually associated with burials - but not in the case of Burnt Mounds).

A cist (pronounced 'kist') is a small stone-lined coffin-like subterranean structure found in Neolithic and Bronze Age archaeological sites. Usually cists contain human remains, either entire skeletons or dismembered bones. They may also contain grave goods, with high status individuals often being buried with fine treasure, ornaments or weapons.
In some places many cists are found together, either collectively under a barrow or cairn, or scattered over an apparent burial ground.
An exception to this inhumation purpose is the cist which forms part of the Meur Burnt Mound on the Orkney island of Sanday which, being part of a Burnt Mound contained no human remains as this was never apparently the purpose of Burnt Mounds - its existence there being something of a mystery.