Many factors affect how a workout feels. It can be difficult to gauge its actual intensity as compared to its perceived intensity. Heart rate fluctuates according to various stress placed on the body in an effort to deliver oxygenated blood to muscles. Because your heart rate is so rapidly adjusting to acute stressors it is a perfect way to quantify the intensity of a workout or individual exercise.
The type and intensity of a workout dictates what type of energy your body will be using therefore it is important not only to train at high intensity but at low intensity as well.
Training at high intensity will push your body into an anaerobic state where you will be gasping for air and your body will use the most readily available energy source: sugars. Our body can only function anaerobically for a short period of time due to limited quantities of glycogen (sugar stored in muscles). Because we have limited amounts of glycogen in our muscles and glucose in our bloodstream, we can only use this high intensity energy for a limited time and will most likely notice a decrease in energy immediately after.
Training at moderate intensity will provide you with a cardio based workout which is usually built around the duration of the workout and will burn mostly sugars for energy but will begin to burn more fat the longer the workout time. Training at low intensity allows you to learn proper lifting techniques and improve form while providing strengthening through a full range of motion. Low intensity exercise is beneficial for those exercising for long periods of time and frequently throughout the week (5-7 days/week). Low intensity exercise does not use as much sugar that we have in our bloodstream (which is vital for daily function), thus requiring less time to rebuild sugar stores after an exercise bout.