Skill mastery
Some abilities within skill groups can be improved by usage. An ability which can be improved could be up to twice as effective when used by a master than used by a novice. The effects of improving the ability depend on the nature of that ability; A damage dealing ability might do twice the damage, a spell effect might last twice as long, or success might be twice as likely.
When training abilities, you have a chance of improving the ability each time you use it. Varying your training and training a group of a number of abilities at once will increase your probability of improving one of them. You can see the level of mastery that you have achieved in a skill group using the skills command. Only abilities which show a percentage mastery next to them can be trained in this manner.