Ashes are an item used to summon Spirits in Elden Ring using the Spirit Calling Bell. You can acquire the bell early in the game by speaking to Ranni the Witch at one of the Sites of Grace at the Gatefront Ruins. You will also recieve the Wolf Ashes, one of the many Spirit Ashes you can acquire.
Spirit Ashes can be equipped to your inventory like other items and require FP to use. There are currently 64 known Spirit Ashes in Elden Ring and all will give you an upper hand in a fight. To summon Spirits using Ashes, you must be in an area that allows for summons. You will be able to tell if the area allows summons by the glowing purple door indicator on the HUD on the left side of the screen.
Many field bosses or large enemy-group areas will allow for summoning Spirits, so it is wise to equip one before a difficult fight and summon a Spirit to help. Many times, the Spirits can take the attention off of you so you can dish out some damage.
Spirit Ashes can be found in many locations around the map, including drops from enemies, quest rewards, in treasure chests, or in other hidden places. Be aware of how much FP each Spirit Ash requires to find the ones that work best with your playstyle and situation.
For more Elden Ring guides, check out How to get Mimic Tear spirit ashes in Elden Ring on Pro Game Guides.