Soul Stones were shards of a Skylander's shattered soul in the previous version of Skylanders: Ring of Heroes used to summon Skylanders and created from the Spring of Souls. Each corresponded to their Skylander and was speculated to be pieces of their being; with more needed the higher the Skylander's rank is.
Soul Stones could also be obtained from the Realm of Souls, Exchange Scrolls or converted using stones obtained from Mirage Tower.
In the Realm of Souls, the shards were to concentrated that they create illusionary copies of the Skylander and alter the field around it to a corrupted version of another area of the Shattered Islands, and the Villains that become playable are illusions created by the Soul Gems; implying that they also hold a character's memories.
On July 7th, 2020, it was announced that during the game's reboot, the Soul Stones will be removed and Skylanders can just directly be summoned.[1]