At the Skill Tree you are able to assign talent points to certain skills or abilities/tactics. Those will give you more talents, such as using medals in battle. You need to assign the points in the right order to unlock new skills/abilities/tactics, but you do not always have to fully unlock a skill/ability/tactic to unlock the next one. As soon as a skill/ability/tactic is unlocked and can be assigned a point, it will get slightly lighter than before. The "Assign Point" button will only be clickable in case the skill/ability/tactic is unlocked and you have at least one talent point.