Your combat Skill Ranking is a measure of how good you are at combat.
It is adjusted when you win or lose fights.
However, only fights in which the opponents are relatively evenly matched will affect the ranking.
For example, if you have 501+ members in your clan and you attack a person with only 3 clan members, you rank will not rise.
The reverse is also true; you do not lose rank for being attacked by someone far more powerful than you.
The exact formula for determining rank increases and decreases is a secret. However, as a general practice, you are more likely to increase your rank by attacking someone with a similar ranking to your own and who has nearly as many clan as you do.
Increases and decreases vary. The smallest increment is 1. The largest increment is... 19?
It is possible to have a negative skill ranking.
Some abilities only become available after a certain rank is achieved.