PoA is a consensus mechanism for blockchain in which consensus is achieved by referring to a list of validators (referred to as authorities when they are linked to physical entities). Validators are a group of accounts/nodes that are allowed to participate in the consensus; they validate the transactions and blocks.
Unlike PoW or PoS, there is no mining mechanism involved. There are various types of PoA protocols, and they vary depending on how they actually work. Hyperledger and Ripple are based on PoA. Hyperledger is based on PBFT, whereas ripple uses an iterative process.