Easy to say, not to practice.
In many situations, even really difficult ones, it is often all that is needed.
But to really just listen means to do so with the full intention of understanding the other person, without interrupting, without getting to your next thing on time, without passing judgement, and without projecting your (needs/stories/beliefs/etc) onto what they are telling you.
This does not mean you shouldn’t respond, but it does mean that you should not react.
Listen, with your whole mind, body (yes – your physical body can listen to), and soul.
Not only does it work to help others, but, paradoxically, you’re far more likely to be heard by others when you do.