Replication slots are a crash-safe data structure which can be created on either a master or a standby to prevent premature removal of write-ahead log segments needed by a standby, as well as (with hot_standby_feedback=on) pruning of tuples whose removal would cause replication conflicts. Slots have some advantages over existing techniques, as explained in the documentation.
In a few places, we refer to the type of replication slots introduced by this patch as "physical" slots, because forthcoming patches for logical decoding will also have slots, but with somewhat different properties.
So now that I've actually gone to use physical replication slots, I can't get them working.