Staking means committing (locking up) a quantity of a cryptoasset to support a proof-of-stake network's operation, directly as a validator or indirectly by delegating to one. In return, stakers typically earn a share of network rewards, roughly analogous to earning interest.
Staking isn't risk-free: staked funds are often subject to lock-up periods, the asset's price can still move significantly while it's locked, and delegating to a poorly run or dishonest validator can result in penalties. "Staking opportunities" promising unusually high, guaranteed returns are also a common scam pattern worth treating with real scepticism.