POSIX ACLs are fine-grained access rights for files and directories. An Access Control Lists (ACL) consists of entries specifying access permissions on an associated object. POSIX ACLs provides more granular control over file and directory permissions than the traditional POSIX permission model.

The traditional POSIX permission model uses a set of file bits to define permissions for the owner, group, and other users. In contrast, POSIX ACLs provide a more flexible and fine-grained access control mechanism by allowing multiple entries in an access control list, each of which specifies a different user or group and a different set of permissions.

With POSIX ACLs, you can grant or deny specific permissions to individual users or groups for a particular file or directory. For example, you can allow a particular user to read and write a file, but deny them the ability to execute it. You can also grant a group of users read-only access to a directory, but prevent them from modifying or deleting any files in that directory.

POSIX ACLs are supported on many Unix-based operating systems, including Linux, BSD, and macOS. They can be managed using command-line utilities such as setfacl and getfacl.

Not all filesystems support POSIX ACLs. Their behavior may vary across different implementations.