The Tree of Life is the skill tree system in EoS. It will allow you to learn new active skills as well as passive traits. Active skills are abilities your character can equip and use. These range from combat moves, to dodges and even magical abilities. Passive traits will grant you stat buffs or unique traits. They require no manual use as they are always active.

Additionally, the tree also contains skill buffs you earn as you use a skill more and more. The tree is split into two parts, the top and bottom halves. The bottom part forms the roots of the skilltree. Each root represents one of the 16 skills in EoS. As you improve each skill you will earn a slight buff to improve that skill. For each skill there are 8 buffs for a total of 128 unlockable buffs. The top half of the tree forms the branches of it. It contains 300 different active and passive abilities. There are 44 active nodes, 64 passive skills and 192 stat nodes to choose from.

The bottom or roots of the tree are unlocked automatically. As you use your skills more and more you will slowly unlock the passive buffs when they level up.
You unlock abilities in the top part of the skill tree by completing quest. This will eventually grant you a skill point that you can use to unlock a new skill. You must start from the bottom of the skill tree and continue on the path. Any new choices must connect to your previous choices.