Apple Inc. has cleverly hidden information on ls in ls' own manual, accessed by man ls.
Grepping for the first + in man ls' output finds this:
Code:
The Long Format
If the -l option is given, the following information is displayed for
each file: file mode, number of links, owner name, group name, number of
bytes in the file, abbreviated month, day-of-month file was last modi-
fied, hour file last modified, minute file last modified, and the path-
name. In addition, for each directory whose contents are displayed, the
total number of 512-byte blocks used by the files in the directory is
displayed on a line by itself, immediately before the information for the
files in the directory. If the file or directory has extended
attributes, the permissions field printed by the -l option is followed by
a '@' character. Otherwise, if the file or directory has extended secu-
rity information (such as an access control list), the permissions field
printed by the -l option is followed by a '+' character.
When you apply permissions to the file using Access Control List (ACL), that + sign appears.
It is usually done by the root user.
An ACL specifies, which user or system has granted access to some object or file.