Relaxation is an aspect of swimming many people do not think about. If you have swum a lot as a child you will naturally be relaxed in the water so you will not have a need to think about it, if you are learning later in life to swim or trying to improve on your swimming at a later stage in your life then chances are you will not be thinking about it as you try to muscle your way through the water.
Have you noticed how many of the best swimmers in the world look so effortless in the water or how some small young kid can shoot past you in the pool with a smile on their face not breaking a sweat? This is because they are relaxed when they need to be relaxed and put in the muscle only where and when it is needed. In particular the freestyle recovery is a part of the stroke that needs very little effort (unless sprinting). Recovering the arm back over into the water should be as relaxed as possible saving as much energy as you can. Another area that needs to be relaxed are the hands, many people will tense and squeeze their fingers together to make a paddle throughout the pulling phase of their stroke. The hands should be relaxed , fingers together but only if relaxed - do not force or squeeze your fingers together, this creates vital energy wastage.
Try adding some slow relaxed swimming into your sessions - do not be afraid to play around with your stroke and exaggerate being relaxed, you may come across something you are in the habit of tensing when you do not need to be tensing.