Restaking lets ETH stakers re-pledge their staked position as economic security for additional protocols - oracles, bridges, data availability layers, app-chains. EigenLayer pioneered the category. Restakers earn additional rewards from those services but accept the risk of additional slashing conditions.
The risk profile is non-trivial. A restaking position can be slashed both at the Ethereum consensus level (for validator misbehavior) and at each AVS (Actively Validated Service) layer it's restaked into. The composability is powerful but the failure modes are still being mapped.