Choosing a Perk
The Experience tab allows you to select a perk for any of your hero's ten levels of experience. Click the level, and then choose the perk in the Selections Sidebar, and hit Save (or Reset to remove the perk). If you choose Attribute, you must select one of the three attributes to improve.
Specializing in a Skill
One of the perks you can choose for a level of experience is Specialize. For non-weapon skills, this allows makes the skill explode on a 5 or a 6. For weapon skills, this reveals a list of tactics you can add to the weapon skill. Make sure to click Save after making your selections.