Skill Points are your "currency" to "buy" Skills. Every time you level up, you are granted a certain amount of Skill Points : Easy Mode : 3 Skill Points per level Normal Mode : 2 Initial Points + 3 Skill Points per level Hard Mode : 1 Initial Point + 4 Skill Points per level Ultimate Mode : 5 Skill Points per level
Each skill requires a certain amount of Skill Point to acquire or level up. Since you have a limited amount of Skill Points, it is recommended to save them and chose your skills wisely.