That's your Ultimate skill slot. Need 12? points in a class skill to unlock.

Yep, just need to level one of your skill lines (excluding weapons) to level 12. This will allow you to use a skill point in that tree's ultimate ability. It's basically your 6th skill on the hotbar, but a more powerful one, They wanted to put it closer to your wasd keys than number 6 would be, so they defaulted it to R. Of course, this can all be rebound elsewhere if you don't like it on the R key.

It isnt a matter of how many skill points you need in the class to unlock it, it is what level your class is. Say I have a dawns wrath templar whos player level is 14 and whos dawns wrath level is 20. The level of the dawns wrath (or whatever ultimate you choose)(only weapons dont have ultimates. guilds, classes, and world magic have ultimate skills) determines when you get your ultimate. Refer to your skill page to find which ultimates you wish to use, and what levels in the classes/guilds/worldmagic you can unlock them. I would only recommend using 2 ultimates to keep cost of skill points down.