What is fatigue a symptom of, if anything?

This is not easy: Fatigue is a very non-specific symptom. It can go with so many different things to numerous to express here, but here are a few possibilities. This in not all inclusive: anemia, blood sugar issues low or high, thyroid disease, depression, sleeping disorders, to name but a few. If you are concerned see your doctor and ask him/her to run some basic labwork to determine a cause if possible.

Many possibilities.: There are innumerable medical, nutritional and emotional conditions that include being tired as a symptom. These range from simple issues to those far more complex. A good checkup and blood testing would be a good starting point.