Simple definition
Linux filesystem ACLs grant named users and groups permissions beyond the traditional owner, group, and other mode bits.
Purpose and use
ACLs, Access Control Lists, grant permissions to additional users or groups without changing file ownership.
How does it work?
Entries extend Unix permissions; an ACL mask can limit the effective permissions of selected entries.
Example and verification
Check effective file access and traversal permissions on its parent directories.
Points to watch
A default ACL controls inheritance when objects are created; it does not automatically rewrite every existing file.
In short
Interpret an ACL together with its mask and path context, rather than only the displayed rwx bits.