A skill tree is an aspect of some roleplaying games, that allow custom configurations of a character's abilities. The Diablo series in particular is well recognized for its use of skill trees and helped popularize the concept.
A skill tree starts with one or several base skills for either a certain class of character, such as a barbarian or amazon, or the player that can become whatever they want starting from scratch.
After base skills, the skills branch out into either more specialized skills or just higher level skills. For a wizard, the base skills can be a basic spell in each of the four elements of fire, water, ice, and earth, for example. The player can then concentrate their spells in one or more of those elements, or focus on learning all the skills but not necessarily being focused in one area.