PBFT is a way for a distributed network to reach the consensus set for the Blockchain even if some nodes are malicious. In Hyperledger, for a transaction to be approved, a certain number of nodes must agree to this. PBFT is the feature of the application to approve the transactions even if some nodes are not approving the transaction.
When a Hyperledger transaction is made, the transaction details are sent to the nodes in the network. There are might some nodes that will approve the transactions and some nodes that won't. The majority (or a minimum specific number) of nodes have to approve the transaction for the transaction to be completed.