Block Producers (BPs) are decentralized entities that produce blocks of the blockchain on EOS - they are similar to what miners are on Bitcoin or Ethereum except that BPs in EOS have to be voted in by token holders. Only the top 21 Block Producers on EOS get to produce blocks; it is expected that token holders will vote in BPs who are technically capable of producing blocks (eg. running reliable servers, ensuring the network is stable) into one of these 21 spots.