What does "+ 3" mean in result of 'ls -al'?

Accidentally, when moving to a mounted volume, and typing ll (alias for ls -laF)

:/media/username/DATA$ lltotal 153drwxrwxrwx  1 username username  8192 Sep  1 20:32 ./drwxr-x---+ 3 root     root      4096 Sep  3 08:14 ../drwxrwxrwx  1 username username 12288 Jul 26 22:29 documents/drwxrwxrwx  1 username username 16384 Sep  2 02:01 downloads/

I have this interesting part: drwxr-x---+ 3

I understand the part drwxr-x---, but what the else part + 3 means?

Ps: I'm using Ubuntu 16.04 x86