Before the battle tower, your skills are limited to level 5. Once you have the battle tower, you can purchase training to raise the level cap, allowing you the option of adding more skill points. Fireball was listed in the skill window as (2/5), and after training would have been (2/10).
Once you have access to the trainer you can learn up to level 10 of most skills (some have a max of 5, like Lockpick). With the first upgrade for the skill trainer platform, you get up to level 13 of skills. With the second platform upgrade, you can raise that to level 15 (29 skills have a max of level 15, 4 of which require level 36 in order to learn the level cap bump).
