All staked validator nodes that were behind the current block in last 10 block window. This list specifically excludes unstaking nodes. These nodes can still perform relays and earn rewards. In fact, they may already be up to speed with current block. However, if they miss 6 blocks in 10 block window, they will get jailed. If that happens, they won't be able to participate in relay sessions, and will not be able to earn rewards.
As of block height: 115658