Something unpleasant is disagreeable, painful, or annoying in some way. No one likes unpleasant experiences.
Since pleasant things are enjoyable in some way, unpleasant things are hard to enjoy. Being in pain is unpleasant. So is being stuck in traffic. It's unpleasant when someone is rude. If your feelings or senses don't like something, it's likely unpleasant. The word pleasant comes from the French plaisir, "to please." If something doesn't please you, it's unpleasant.