This article explains what a BSA file is and how to open one on your computer.
How to open BSA files: Windows Magic
A file with the BSA file extension is a BSARC compressed archive file. BSA stands for Bethesda Software Archive.
These compressed files are used to store source files for Bethesda Softworks computer games, such as sounds, maps, animations, textures, models, and so on. An archive makes organizing data much easier than having it stored in dozens or hundreds of separate folders.
BSA files are stored in the /Data/ folder of the game's installation directory.