smgr is a commonly used abbreviation for "storage manager" in the source code.
Source code
See: src/include/storage/smgr.h
/* * smgr.c maintains a table of SMgrRelation objects, which are essentially * cached file handles. An SMgrRelation is created (if not already present) * by smgropen(), and destroyed by smgrclose(). Note that neither of these * operations imply I/O, they just create or destroy a hashtable entry. * (But smgrclose() may release associated resources, such as OS-level file * descriptors.) * ... */