What is Anchor Peer in Fabric?

A peer node on a channel that all other peers can discover and communicate with. Each Member on a channel has an anchor peer (or multiple anchor peers to prevent single point of failure), allowing for peers belonging to different Members to discover all existing peers on a channel.

You can read about this in glossary on hyperledjer fabric web site:

https://hyperledger-fabric.readthedocs.io/en/release-1.1/glossary.html#anchor-peer

Anchors peers encoded into config update transaction in order to enable communication between peers of different organizations and discover all active participants of the channel.