What is it?
When a position is liquidated, the liquidator repays some debt and receives collateral with a bonus. That bonus is extra collateral the liquidator keeps as incentive to do the work.
Why does it matter?
A larger bonus makes liquidations more attractive to searchers. For the borrower it is an extra cost on top of losing collateral.
Example
If the bonus is 5%, a liquidator who covers $10,000 of debt can receive about $10,500 of collateral (subject to Aave's close factor and bonuses).
Where can I find it?
Asset detail, Reserve configuration, Liquidation bonus. Shown as the extra percent a liquidator can receive. Live Activity and Liquidations show LiquidationCall events in the current window.