Training hard and regularly without having appropriate rest can have a detrimental effect on your performance and even make you ill. Every training session results in a small degree of damage to your muscles, nerves and connective tissues. It also fatigues your energy systems and can leave you depleted. Rest days are important in helping to restore energy levels and repair and rejuvenate those parts of the body that get damaged during training sessions.
If you are a really driven individual you might think "Yeah but if I train in the morning on Thursday and evening on Friday that's over 24 hours rest!" - but the mistake here is to underestimate the importance of sleep for recovery. Try thinking of a rest day as a "double sleep" instead and you'll get a much truer picture of what you need. Taking this time out from your training will allow you to get back to the pool feeling energetic and ready to keep working towards your goal.
You can access our full stretching routine on our Swim Smooth Guru (subscription required) which would be a perfect addition to your rest day: