Empathy is the ability to see things from someone else's perspective.
You can empathize without sympathizing: I understand your frustration, but I don't feel the same way.
You can empathize without being nice: You're feeling angry right now, but don't you dare take that anger out on me.
You can empathize without agreeing: You feel that you were treated unfairly, but let's take a look at things from another perspective.
People feel much more validated, and open for change, when you empathize.