Plain meaning
Start with the shortest useful explanation before going deeper.
A discount on collateral that incentivizes liquidators to repay the debt of unhealthy positions in lending protocols. When a position's health factor drops below 1, liquidators can repay a portion of the debt and receive the equivalent collateral value plus a bonus (typically 5-15%). The bonus must be large enough to cover transaction costs and risk, but not so large that borrowers suffer excessive losses.