Subform of PoS, in which token-holders can still participate in the security and maintenance of a blockchain without having to actively operate a node. In BPoS systems, token-holders delegate the validation-rights contained in their token to a validator of their choice, increasing the respective validators delegated stake and ultimately increasing its' likelihood of validating more blocks.
This delegation is usually non-custodial, so token-holders remain the sole owners of the token itself. Both, validators and their delegators are subject to slashing in BPoS systems. In most BPoS networks, the validator set is fixed. Hence the amount of validators eligible to participate in the maintenance of a blockchain is pre-defined (oftentimes this value can be change through governance).
Blockchains are run and maintained by a decentralized, globally distributed network of nodes and not a central entity. Hence, a mechanism is needed that helps this network, often-times consisting of unknowing or untrusting parties, to establish security, validity, and guaranteeing availability. Currently, there are two main mechanisms, namely Proof-of-Work and Proof-of-Stake, that are used in blockchain networks to provide for security through economic incentives and cryptography. Furthermore, these mechanisms help to appoint the next node which gets to add a new block to the blockchain.