Ashes are a junk item in The Witcher 3: Wild Hunt.
It is very common as loot everywhere, although typical to find around burned down buildings and on non-enemy corpses. It can be dismantled from a lot of other junk or discardable quest items, and it is sold by the Merchant in Midcopse.
In the Blood and Wine expansion, it is also used as an alchemy ingredient for crafting gray armor dye.