Every form has a level, which is an approximation of both its power, as well as the exertion necessary to perform it. There are five levels of forms. Basic forms are those which can be easily initiated at any time without needing to spend ki points. Other forms range from Sho-level to Ga-level, all the way to Sen-level, the most powerful forms in the world. The higher a form's level, the higher level a lacer must be to use that form.
Form level and character level don't correspond directly. The level a character needs to be to initiate a form of a certain level will be covered in that class or subclass's description.