A stake pool is a reliable server node that focuses on maintenance and holds the combined stake of various stakeholders in a single entity. Stake pools are responsible for processing transactions and producing new blocks and are at the core of Ouroboros, the Cardano proof-of-stake protocol.
To be secure, Ouroboros requires a good number of ada holders to be online and maintaining sufficiently good network connectivity at any given time. This is why Ouroboros relies on stake pools, entities committed to run the protocol 24/7, on behalf of the contributing ada holders.
While Ouroboros is cheaper to run than a proof of work protocol, running Ouroboros still incurs some costs. Therefore, stake pool operators are rewarded for running the protocol in the form of incentives that come from the transaction fees and from inflation of the circulating supply of ada.