If you're looking for results its no secret, you have to train hard. This means lifting heavy, performing HIIT workouts, full body functional exercises based around movements etc. You need to put a demand on your body that it's not always quite comfortable with in order to progress and improve. However, all this demand can take its tole on the body. If you're going to train hard, you have to recover and you need days/workouts that are dedicated to that specifically. Recovery days are important to help repair damage done to the body during your workouts, stretch muscles and prevent imbalances, increase mobility aiding in injury prevention etc.
There are plenty of options available for recovery after hard training days. Depending on your financial situation, location, or availability of services you may choose one over the other. Massage should be first on everyones list of recovery options. When you're training hard, muscles become tight, injuries can occur, and poor posture may result and lead to further problems in the future. Regular massage will help prevent and treat all these issues. If you can't afford regular massage sessions than foam rolling or using a PVC pipe may be a better option as a form of self massage.
Yoga is also an excellent option for your lighter or recovery days. It's a chance to stretch out the entire body, work some muscles you may be neglecting in your regular training, and still feel like you've had a small workout. I would highly suggest taking a restorative or lighter yoga class if you're already training hard. This means no power yoga or ones specifically designed to be a workout. I would suggest trying Yin Yoga or a light Hatha class.
Another great form of recovery is swimming, or rather floating. Simply floating in water takes a lot of pressure off the joints of your body, and it's also very soothing. If you have access to a pool try using some "pool noodles" to just float for a while and relax. Bring a friend for conversation or just close your eyes and enjoy the time alone to reflect.
Regardless of what you choose try to incorporate some form of recovery (or a couple forms) during a few days of the week. Trust me, your body will thank you for it!