ACL stands for access control list. ACLs extend permissions on files beyond the traditional user-group-others triple.
Many Unix variants provide more fine-grained file permissions than the traditional, standard triple of permissions for the owning user, the owning group and others. These permissions are expressed through access control lists. The permissions that can be expressed through ACLs vary between Unix variants.
POSIX ACLs allow setting the read/write/execute bits separately for more than one user or group, and also allow setting default access rights for new files at directory-level, with default ACLs.