Rest day is as much a part of your training as your training is.
Very often it is assumed that a rest day is needed due to tiredness or, but trainers know that rest is where the magic happens.
It’s during your rest time that you become fitter and stronger.
When you have a tough training session, the muscle is broken down which is why you will be sore the next day, and most likely the day after that too. The muscle is clever and “remembers” the breakdown and draw on the protein you eat to repair itself stronger than before. So it’s a case of breaking it down, to build it up stronger, and repeat.
The key here is to allow the muscle enough time to repair. If you do a hard legs day and your quads or hamstrings are sore afterwards, whether you work out of not the next day, go easy on those muscles and allow them to do their thing. Continually battering the same muscle will just break it down without allowing it to rebuild and become stronger.