You get points for posting and getting votes from posts and comments. levels are based on the total count.
Level^2*10 = total points
So a level 4 has (4x4x10) 160 points, and a level 6 has (6x6x10) 360 points. You can make a sub for every level you rank up, and at level 6 the captchas go away.
Put the bubble between the lines, it's rather intuitive if you're not a moron.