Clicking anywhere on this section except on the + sign will open the character class selection screen
The 3 plus signs are used to check for class prerequisites.
The first + sign will give your character the class prerequisites based on the first class you selected for your character. The same is true for the second and third + signs, the will respectively give prerequisites for the second and third class of your character.
Some prerequisites are not given to the character even if missing, this is the case of the base attack bonus. Modifying is for the character would cause some incongruity in the database. You will have to raise the level of your character for the base attack bonus to meet the requirements.