By staking your SOL tokens, you help secure the network and earn rewards while doing so. You can stake by delegating your tokens to validators who process transactions and run the Solana network.

Read more at https://docs.solana.com/staking

Here's what Solana Docs have to say on Stake Pools: "This on-chain program pools together SOL to be staked by a manager, allowing SOL holders to stake and earn rewards without managing stakes. Users deposit SOL in exchange for SPL tokens (staking derivatives) that represent their ownership in the stake pool. The pool manager stakes deposited SOL according to their strategy, perhaps using a variant of an auto-delegation bot as described above.

Your represents your share in the pool’s total stake, which grows constantly as rewards are accrued. This reflects in the -SOL exchange rate which will be used to calculate the SOL you get back when you unstake. Total price growth is equal to the pool’s APY.

Read more at https://docs.solana.com/staking

Any user who delegates their SOL to a Validator (either directly or via Stake Pool), enabling them to validate new blocks and keep the network alive, becomes a Delegator.
Anyone who has some SOL in their wallet may delegate it – and earn rewards by doing so