By choosing to test out of a number of skills at a given point in your tree, you "unlock" the next lessons. You can't unlock all of the course at once though, unless you were to test out to the trophy.

Also, when you finish a skill è.g. Basic1. Then the next lessons in the following skill become unlocked. It is gently guiding you through the tree but if you want to skip a group of skills there's several points where you can test out your knowledge and unlock a whole bunch of skill as ColinIsCool23 suggests.