TL;DR
Restaking lets you reuse already-staked SOL (or a liquid staking token) to help secure additional services — like oracles or bridges — earning extra rewards on top of base staking yield, in exchange for added risk.
Normally, staked SOL only secures the Solana network and earns inflation rewards. Restaking takes that same staked capital and commits it to securing additional services — such as oracles, bridges, or other networks — that pay their own rewards. You earn a second layer of yield on top of base staking, but your stake now backs those extra services and takes on their risk.
Liquid staking stakes SOL and gives you a liquid token (mSOL, jitoSOL) that earns standard validator rewards and stays usable in DeFi. Restaking goes a layer further: it commits your stake to securing extra protocols for additional yield. Some restaking designs build on liquid staking tokens, so you can restake an LST you already hold.
Restaking stacks risk: on top of normal staking, you take on the smart-contract risk, misconfiguration risk, and any slashing or penalty rules of every additional service your stake secures. Higher layered yield comes with higher layered risk, so understand exactly what your stake is backing before committing — and treat it as an advanced strategy.