An individual or group who has hardware chosen to produce the next block of a blockchain. This is utilized with Proof-of-Stake (PoS) consensus mechanisms. Ethereum is the most famous PoS network.
In Delegated Proof-of-Stake (DPoS), users are allowed to vote on block producers to validate transactions and generate blocks. Hive is the most active DPoS blockchain.
Block producers are decentralized entities.
With Proof-of-Work chains, the block producers are called miners. Bitcoin is the undisputed leader utilizing this mechanism. Litecoin is another well known, long time PoW network.
Block producers are incentivized to generate blocks and maintain the network. On many chain, this entails ensuring the ledger is free from any errors, including double spend problems. Blockchain, at its most basic level, enables the transfer of value, through either coins or tokens, in between wallets.
They are the ones who run the software that allows that to happen.
Ledger maintenance is the same function that banks serve in the traditional monetary system. The difference is that blockchain is decentralized and a properly functioning system should have block producers without any attachment to each other.