To add, basically insecure souls are dropped when you died, meaning that everyone in the game can pick the orb with the insecure souls. Let's say you have 3000 souls + 1000 insecure souls, when you die you will have 2000 souls, and a orb with 1000 souls will be dropped as loot. You can pick your own orb if nobody pick it. and as Crusty says your 1000 souls are going to drop up to 0, once the number reach 0, you will have those 1000 secured, meaning that if you died you will have 3k of souls and no orb dropping.