What is a Bitcoin mining pool?
A group of miners who share the work of finding blocks and split the rewards, so each gets paid steadily instead of rarely.
Mining alone, one small miner might wait years between blocks. A pool combines many miners' hashrateHashrateHow many guesses a second all of Bitcoin's miners make together. More hashrate makes the network harder to attack. and pays each a share of what it earns, in proportion to the work they contribute.
The pool operator builds the block candidates and coordinates; the miners still own their machines. A few large pools account for much of the hashrate, which is why pool concentration is watched as a decentralisation risk.