Detailed informaiton regarding the skills can be found at [Menu] → [Skiils].

You can expand Active skill slots up to 5 with Caret.
And you can pick your skill of choice.

Passive skills improves your power slowly but permanently.
Each skill can reach up to level 20, Skills are for enhancing resistance by attribute, and improving stats of the character.

You can modify or register a new skill from skills menu.

[Daily Dungeon] Abyss

To make the skill more proficient, you need to improve your skill first.
You need more ether and extra Gold to level up your skill.
You can collect ether from daily dungeons, quests, guild mission and achievement, etc.