This refers to the maximum share of staked assets attributed to a validator (own & delegated stake) that can be slashed in case the validator signs more than one block at the same time. Double signing is more severely punished than being unavailable as it poses a bigger security threat to the network. In some networks (e.g. Lido), the slashed amount is 'socialized' across all delegators. This means that instead of a few parties suffering a big slash, every participant gets slashed by a small amount.