The skill tree allows you to spend skill points to become a better shaman! There are three areas you can train: Spiritual guide, Wind master, Mechanician. The skill tree can be accessed by clicking "Skills" in the menu or by the yellow exploding number icon that appears at the bottom left of the game when their are points available to be allocated.
You can earn skill points by leveling your mouse. Each level gains you a skill point. Experience must be gained to level. Getting cheese and returning to the mouse hole and saving mice while your shaman gains experience. Each level requires a certain amount of experience.
On the skill tree, you can redistribute the skill points you allocated by clicking the "Redistribute" button. However, redistribution will use cheese, requiring one cheese for every skill point obtained.
Each row of the tree is unlocked for every 5 skill points you allocate in that area. There is no restriction on how the points need to be allocated within each area, so long as it is in multiples of 5.
The second row is unlocked when 5 points are allocated to the first row.
The third row is unlocked when 5 more points (10 total) are allocated to the second row or before.
The fourth row is unlocked when 5 more points (15 total) are allocated to the third row or before.
The fifth row is unlocked when 5 more points (a total of 20) are allocated to the fourth row or before.
The last row is unlocked when 6 or more points (a total of 26) are allocated to the fifth row or before.
To get to the last tier quickly, you would need at least 27 levels.