Mining pools are cooperatives that aim to smooth out expected revenue by pooling the mining power of participating miners. In return, they usually charge you 0-5% of your mining rewards. The mining pool submits blocks with proof of work from a central account and redistributes the reward to participants in proportion to their contributed mining power.
Warning: Most mining pools involve third party, central components which means they are not trustless. In other words, pool operators can run away with your earnings. Act with caution. There are a number of trustless, decentralised pools with open source codebase.
Warning: Mining pools only outsource proof of work calculation, they do not validate blocks or run the VM to check state transitions brought about by executing the transactions. This effectively make pools behave like single nodes in terms of security, so their growth poses a centralisation risk of a 51% attack. Make sure you follow the network capacity distribution and do not allow pools to grow too large.