A filesystem boundary is the intended limit of where a program may read or write files. It defines which paths are safe, which directories are off-limits, and whether a process may create, overwrite, or delete content outside its expected workspace. In secure software, that boundary is a trust control, not just a path check.
It matters because many attacks try to cross it. Archive extraction, file uploads, backup restores, and path-handling bugs can let attacker-controlled names escape a temporary folder and reach sensitive locations such as configuration files, credentials, or application data. Defenders protect this boundary by validating paths, rejecting traversal sequences and unsafe symlinks, extracting into isolated directories, and running with minimal permissions. When a program treats untrusted input as instructions for where files should go, the filesystem boundary becomes a direct security target.