Fatigue is a reduction in the amount of energy that one is able to expend and goes hand in hand with a lack of motivation. While physical and mental fatigue are often found together, they're distinct as well. This page contains information on both. It's also helpful to distinguish between acute and chronic fatigue. Acute fatigue is normal and derives from doing tiring activities such as exercising, staying up later than usual, or doing mentally demanding work. Chronic fatigue is when, regardless of your activities, you're generally fatigued and lacking motivation. This isn't normal and suggests that something is wrong. The causes of chronic fatigue are numerous and include insomnia/poor sleep quality, stress, anxiety, depression, metabolic abnormalities, severe nutrient deficiencies, use of drugs, and a lack of exercise, among many others.
Supplements on this page may work through ameliorating the various sleep and mood issues that can lead to fatigue, by being neuroprotective (as much fatigue is due to neural dysfunction) affecting metabolism, hormones, and neurological function, among other roles. For this reason, simply because a supplement can reduce fatigue in some instances doesn't mean that it will work for everyone.