Spending Skill Points in hero's Skill Tree grants boosts to the player's overall basic abilities.
The more points you spent on a single skill, the stronger it becomes. Points you need for learning a skill once will be shown on the "Learn" botton, and the number gets higher when this skill is already high-level. When you have maxed out the number of points on a skill, the skill is mastered by your hero.
Skills further down the page are relatively stronger, but they require some skills located before them in the Tree to unlock. Tap a locked skill to check out what skills it requires before you unlock it.