Why is Tension so bad?
When your muscles or joints are tense, they can't move so easily. To play smoothly, without bumps, your movements should be easy and flexible - which you can't do when you are tense.
Playing fast passages can be more difficult when you are tense. Your fingers are held back by the tension and can't move freely. As a result, they do not get to the correct places on time. This makes for bumpy fast notes - some longer, some shorter, some stronger, some weaker.
What does it feel like to be tense?
Sometimes it is hard to describe what tension feels like. It is the feeling of unease, tightness, and being crunched up.
Good Posture
You should double-check your posture and seat height. If you do not start out with the right setup, you might be moving your body in uncomfortable ways to reach the keys and pedals.
Practice Relaxing while Playing
Try playing some easier pieces and notice whether you experience tension in any of these places. As you practice each day, remember to pay attention to tension. You might experience more tension when playing a new piece or a particularly difficult section. However, if you do not learn to relax, you are actually making it harder to play. Every time you find tension as you play, work on relaxing those muscles.