What is single member blockchain? I just heard about single member blockchains like quorum and hyperledger-fabric. So what does single member mean? Is this some kind of restriction or what?
The term single member is not correct. Hyperledger blockchains, such as Hyperledger Sawtooth or Hyperledger Fabric, are private blockchains or permissioned blockchains. You need permission to join the blockchain network. They are usually setup by consortiums (an example would be to track fisheries or other marketplace goods as it moves from harvester or producer to wholesaler to market to consumer).

Since the blockchain is permissioned, it does not usually use digital currency to function.