File archive created with LU, an older file archiver for MS-DOS; stored as a library of files and not compressed.
The LBR file extension is associated with EAGLE (short for Easily Applicable Graphical Layout Editor), a CAE program for creating PCB schematics, developed and sold by CadSoft Computer, Inc. A .lbr file contains data of a single library, which can contain various packages, devices or symbols.