Climb a tree is an int/adv Pilates reformer exercise. Like most Pilates exercises, this is a full body exercise. It specifically works the abdominal muscles and the back extension muscles. It can be a hamstring a stretch as well. The asymmetry of having one hip flexed and one extended will give you an extra control challenge for the abs in order to keep the pelvis stable.
Climb a tree is done with the long box turned sideways on the reformer with the back edge over the shoulder blocks. You will need enough spring tension to keep the carriage stable.
The exercise instructions here are a good reference for the choreography of the exercise. However, it is strongly recommended that you support your practice by taking classes with a Pilates instructor who can give you personalized feedback.