A filesystem baseline is a known-good inventory of expected files, directories, permissions, and sometimes hashes for a system. It gives defenders a reference point so they can quickly spot added, removed, renamed, or modified items that should not be there.
In cyber security, baselines matter because many attacks hide in the filesystem rather than the user interface. A malicious WordPress must-use plugin, for example, can live in a trusted directory and load automatically on every request while remaining outside the normal plugin list. If defenders compare the live filesystem against a baseline, unexpected PHP files, odd directory names, or changed timestamps stand out during incident response. Baselines also help validate cleanup after malware removal and reduce reliance on dashboards alone.