This will be a short post, because while heart rate is important, there's not much to say on it.
After working out and getting my heart rate up I feel like I can breathe so much easier. Like there's more room in my lungs. Getting your heart rate up is important because it allows your lungs and heart to function best. Your target heart rate vary's depending on your height, weight, and age. There are a several different ways to find your heart rate, and I suggest you look them up. Knowing your heart rate is important because it tells you when you are working your hardest and also when you need to slow down.
By the way, you may remember all this if you've ever had a highschool gym class :)