Skills play huge role in your playstyle and changes according to your class. Each class has 3 active, 1 passive and 1 ultimate skill.

Skill Bar

First 4 slots of default minecraft hot bar are reserved for skills. Each class has 5 skills but 1 of them is passive so you see 3 active and 1 ultimate skill on your hotbar. Press 1-2-3-4 keys to cast skills. You can see the cooldown after casting a skill.

Skill Points

Skill points are used for learning or leveling up skills. Min skill level is 0 which means not learned while max skill level is 4. Skills points are earned by leveling up your class. So you have different skill points for each class. You can fine more about classes here:

Unlocking Skills

You can open menu by clicking the Menu Book in your inventory. (Open your inventory with E key) At the menu click Character and then Skills. Here you can see your available skill point on GUI Name. Left Click on a skill to spend points on it or Right Click to take your points back.