Restaking is the practice of using already-staked assets (or liquid staking tokens representing them) as economic security for additional protocols, beyond the base blockchain they're staked on. EigenLayer popularized the model on Ethereum, letting stakers opt in to secure oracles, bridges, data-availability layers, and other 'actively validated services' (AVSs) in exchange for extra rewards.
The appeal is capital efficiency: the same staked ETH that secures Ethereum can simultaneously back the security of other systems, rather than each new protocol needing to bootstrap its own separate validator set and token from scratch.
The cost is stacked risk. Restaked capital is subject to Ethereum's own slashing conditions plus whatever additional slashing conditions each AVS defines. A bug or malicious action in any service you've opted into can put your restaked principal at risk, even if Ethereum itself is functioning perfectly. Liquid restaking tokens (LRTs) like weETH and ezETH add a further layer of smart-contract and operator risk on top of that.
Restaking is still a young and fast-evolving part of the staking landscape; treat higher advertised yields here as compensation for meaningfully higher, harder-to-quantify risk rather than 'free' extra return.