You just finished your last set, and you grab your phone and water bottle and head straight out of the gym. That was it, you thought you had done everything right, you warmed up well, did some cardio, even hit a PR, yet you forgot an essential part of your workout: cooling down. 
Stretching and doing other exercises to cool down after an intense workout is crucial, and is something I am guilty of never doing. The 5-10 minutes you spend relaxing your muscles after a workout can save you from a whole bunch of muscle soreness and cramps a day or two later. This isn’t to say that you won't be sore, but instead that its severity would be reduced. Cool down exercises bring your body back to its pre-exercise state gradually, instead of the surprise your body gets when you go from running to sitting on your couch. This gradual change has been studied to be associated with a lower risk of injury as well as an improved mood since it allows the mind to go from its alert to its relaxed state.
Cool-down exercises are often considered to be just a less intense version of your actual workout. For instance, if you went on a run, brisk walk for a few minutes to cool down, or if you trained your chest, then drop the dumbbells to a very low weight and repeat the movement. 
So that’s it, don’t forget to adequately cool down after you exercise. Now go out there, and crush it!