This error code means that the paymaster or aggregator used by the user operation does not have sufficient stake with the EntryPoint.
User operation’s storage access rules prevent them from interfering with each other. But “global” entities - paymasters, factories, and aggregators are accessed by multiple user operation, and thus might invalidate multiple previously-valid user operations.
To prevent abuse, ERC-4337 bundlers throttle down (or completely ban for a period of time) an entity that causes invalidation of large number of user operations in the mempool. To prevent such entities from "sybil-attack", bundlers require them to stake with the EntryPoint, and thus make such DoS attacks very expensive. Note that this stake is never slashed, and can be withdrawn any time (after unstake delay).
Updated about 1 month ago