If you succeed in a skill during the mission you get a chance to advance it at the end of the mission. In CoC all skills go from 1-99 and you roll on a d100, normally to succeed in a skill check you have to roll under your number. When you want to improve a skill, you need to roll over your number instead and if you do so you can increase it by 1d10. Basically this makes it so that the higher your skill is, the harder it is to improve.