Skills are defined by special affects that increase your abilities and can be equipped after acquiring them. There are generalized skills that increase your damage dealt to stunned enemies or your attack based on the number of territories you own, and there are special skills that only work for specific characters (and are characterized by either a note in the description or by the character portrait being beside the name).
To acquire the skills, you have to play Bingo by collecting coins from drops, special conditions, or through treasure chests. Most of the early Bingo skill notes simply require a fresh coin (Level 1) to meet the conditions, but you will find that some of the later end notes will require higher levels (Max level) or higher grade (three grades: bronze, silver, and gold). To check on your status on the Skill Notes, choose an episode and access the Skill Notes option, and it will show you a nine coin spread for that specific note and what skills it will reward you.