the ability to behave without causing embarrassment or attracting too much attention, especially by keeping information secret

"Can you trust him with this?" "Yes, he's the soul of discretion (= he will not tell other people)." “在这件事上你能相信他吗？”“当然，他可是个守口如瓶的人。”

the right or ability to decide something

Students can be expelled at the discretion of the principal (= if the principal decides it). 校长有权开除学生。

I leave the decision to your discretion (= for you to decide). 我让你来决定这件事。

discretion is the better part of valouridiom saying 勇敢者贵在审慎。