Ability for a group of proof of work Miners to come together and form a single more powerful miner.
Similar to what Delegators or Nominators do in proof of stake networks.
Centralizing Effect
As networks get bigger and bigger, it is quite useful to have Mining Pools and Delegators or Nominators to allow smaller participants. But, it is also important to be aware that if the mining pool (or the proof of stake operator) has excessive control over the entire pool, it could lead to the Trustlessness of the system being compromised.
Imagine if a mining pool can use the power of millions of people, but then propose an invalid block to the Bitcoin network. This could lead lead to that mining pool suddenly being able to control a Byzantine Threshold of the system’s Economic Security.
There are many websites that track the centralizing effect of staking/mining pools in Bitcoin and Ethereum.