Skills are specialized attacks unique to a monster which they unlock as they level up. Each new one is more powerful than the last and require using more energy. The first and third skills are elemental attacks which Tend to cause more damage to other types of monsters. Gems can also further power skills as each skill may have one gem attached to it, however, gems were removed form the game in a recent update.
When an opponent uses a skill on your Mino, this may result in a possibility to block the attack.