This refers to the maximum share of staked assets attributed to a validator (own & delegated stake) that can be slashed in case the validator is offline/unavaiable for a certain time period. This time period is defined by the protocol and differs for each project. 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.