On the Tree of Contemplative Practices, the roots symbolize the two intentions that are the foundation of all contemplative practices. The roots of the tree encompass and transcend differences in the religious traditions from which many of the practices originated, and allow room for the inclusion of new practices that are being created in secular contexts.
The branches represent different groupings of practices. For example, Stillness Practices focus on quieting the mind and body in order to develop calmness and focus. Generative Practices may come in many different forms but share the common intent of generating thoughts and feelings, such as thoughts of devotion and compassion, rather than calming and quieting the mind. (Please note that such classifications are not definitive, and many practices could be included in more than one category.)
You can learn more about the Tree of Contemplative Practices here.