A skill or ability requires ranks to level up. Each level requires a different number of ranks to level up, starting at 10 and increasing by 5 each time it is levelled. When it levels, the ranks for that ability go to one, and the required ranks for the new level are put in place.
Example:
Sword Mastery Lvl 1:
*insert description here*
Current rank: 1
Needed to Level: 10
+9 ranks
Sword Mastery Lvl 2:
*description*
Current rank: 1
Needed to Level: 15
I hope this makes sense.