Skill Points
Skill Points are used to level up your Heroes' abilities and actually use them in battle. When you level up a heroes' skill, the cost for each level jumps quickly. This is also similar when you actually use skills in battle. Using a skill for the first time in a battle will cost relatively low. However subsequent uses shoots the cost even higher. Almost exponentially. So think carefully on where and when you decide to use their skills.
You can obtain skill points from the Daily Log in, event rewards, randomly from landing zone rewards, daily Survival participation reward, and from destroying structures in PvP, etc..