To better understand endurance, it is useful to know something about what energy is and the body's energy systems.

Our energy is obtained from macronutrients (fats, carbohydrates, and proteins). Our body breaks down these macronutrients and converts them into phosphates. Adenosine triphosphate (ATP) and creatine phosphate (CP) are the phosphates that give us energy. These substances drive the vast majority of all body processes (Picture 1).

Broadly speaking, there are two ways to form energy (phosphates), anaerobically (= without oxygen) & aerobically (= with oxygen). We will spend another moment going into the exact processes that ensure ATP is eventually formed.
An important feature of forming energy without oxygen is the formation of lactic acid. When lactic acid builds up, our muscles become tired (acidified).

Our endurance is divided into the same forms of energy, anaerobic & aerobic. Anaerobic endurance is endurance for short-term activity at a very high intensity. This involves low-oxygen use where lactic acid is formed. Aerobic endurance is endurance for workloads lasting longer than 2 to 3 minutes. These energy systems slowly flow into each other, as visible in Table 1.

Once we start training, there are different zones of endurance. These zones are divided from Zone 1 to Zone 5. In Table 2, we have listed the zones, the percentage of oxygen expressed as VO2 max, the percentage of maximum heart rate, lactic acid accumulation, and the typical accumulated duration within the zone.
Zone 1: Moderate activity like walking or warming up
Zone 2: Weight control/Fat burn: Incline treadmill cardio, weight lifting, and fitness
Zone 3: Endurance cardio training (Aerobic)
Zone 4: Heavy cardio training (Anaerobic): Interval training
Zone 5: Maximum effort: Sports tests, interval training

Reference list
de Morree, J. J. (2017). Inspanningsfysiologie, oefentherapie en training (3rd ed.). Bohn Stafleu van Loghum.
Seiler, S. (2010). What is best practice for training intensity and duration distribution in endurance athletes? International Journal of Sports Physiology and Performance, 5(3), 276-291. doi: 10.1123/ijspp.5.3.276.