What is the "+" in "drwxrwsr-x+"

From info ls, under the What information is listed? section, regarding the output produced by -l:

 A file with any other combination of alternate access methods is
 marked with a '+' character.

Generally, it means it has an ACL set.

As stated by @MadHatter this means the File/Directory has additional right trough Access Control Lists. Usually the Owner:Group system is enough, but in some cases you need a fainer grained permission control. There comes the acl system in touch.

To see the acls on a specific file/dir simply type:

getfacl myfileordir

For changing the permissions use the setfacl command. See in the man page of it, for the proper syntax.