At the start of a turn, if the last one you got enought exp to level, all of your 'villagers' level up gaining points in 1 skill, this happens after you get nel people/childern grow up
The skill that is improved is chosen at random but used a formula that lookes like and is broken down like this.
70% chance to raise class skill (for my Fat Rat this is Health 31 & Strength 15)
20% chance to raise a learned skill (for my fat rat this is Craft 2, Dexterity 1, Distraction 1, gathering 1, intelligence 2, perception 5, ranged damage 4, shielding 1, stealth 2, will 6)
10% chance to raise any other skill (where for subsequnent level-ups it becomes a learned skill)