They're lil things that people can add to make their posts more personal, whether you put an image or text in your sig is up to you.
Some more specifications and stuff can be found here:
https://smgclan.net/misc.php?action=help&hid=17
And as Masty said you can DM him if you'd like one made, or post in the Sig Request Forum! :D
Great answers guys, gonna lock the thread. :P Feel free to DM me if you're still confused about anything.
*flails*