Discernment is the ability to make a smart judgment about something. If you're voting for Student Council President, you need to use discernment to pick the best candidate.
The noun discernment describes a wise way of judging between things, or a particularly perceptive way of seeing things. If you can understand something that's somewhat hidden or obscure — if you figure out the themes of a confusing movie, for example — you're using discernment. It comes from the Latin word discernere, meaning "separate." Discernment separates what's important or true from what's not.