If you have the freedom to decide something on your own, the decision is left to your discretion. You're in charge.
Discretion traces back to the Latin verb discernere, "to separate, to discern," from the prefix dis-, "off, away," plus cernere, "separate, sift." If you use discretion, you sift away what is not desirable, keeping only the good. If you have the freedom to choose, something is "at your discretion." Watch out when you hear the phrase "viewer discretion advised" on TV or at the movies — you will most likely encounter profanity and violence.