what does the + mean? How can it be removed?

Let's say we have file permission like -rw-r----- or drwxr-xr-x now question about that, it's well known and clear. But, what does it mean if we have: -rw-r----- + or drwxr-xr-x + ? from where the plus sign is comming? It's not a suid or a stiky bit, what is it? What function is related to it? From where is it comming and how can it be removed? For instance doing some thing like: chmod 0640 doesn't help!