Also called: rmgr, RmgrTable entry, WAL resource manager
In plain English
A resource manager is a subsystem that knows how to write and replay its own kind of WAL tuple/" title="Tuple">record. Heap, B-tree, transaction commit, sequences, and more each register one in the RmgrTable, providing callbacks — most importantly a redo function used during recovery and a desc function used to print the record. Each WAL record’s header carries an xl_rmid that routes it to the correct manager.
Why it matters
This modular design is why recovery is reliable and extensible: there is no single monolithic replay routine. When you read pg_waldump or pg_walinspect output, the resource-manager column tells you which subsystem produced each record — invaluable for finding what is generating your WAL volume.