A libref is a temporary name associated with a SAS data library. A libref must be associated with a SAS data library before that library can be used.
Since a libref is temporary (it no longer exists when your SAS session ends), at different times you can use a different libref to refer to the same SAS data library.
Librefs can be assigned with LIBNAME statements. See SAS Language: Reference for more information.
The SAS System automatically assigns two librefs for you: WORK, which is associated with a temporary library, and SASUSER, which is associated with a permanent library.