Discretionary means it's up to you to decide. Bathing regularly is a discretionary act, so you can opt not to do it if you want; note, however, that your friends might disagree with us on this.
Discretionary is often used to describe money that isn’t designated for a particular purpose. It’s up to the people in charge to decide how discretionary funds should be spent. Discretionary comes from the word discretion, which can be used to mean “the right to decide something based on one’s own judgment.” If you're given a task to complete at your discretion, you can decide how you want to do it — or whether you want to do it at all.