Google says, "This package contains the libacl.so dynamic library which contains the POSIX 1003.1e draft standard 17 functions for manipulating access control lists."
acl (tools for using POSIX Access Control Lists)
This package contains a set of tools and libraries for manipulating
POSIX Access Control Lists. POSIX Access Control Lists (defined in
POSIX 1003.1e draft standard 17) are used to define more fine-grained
discretionary access rights for files and directories.