I think it's when the yeast in your body starts to die off (as you start to eat better) and you feel really fatigued because of it. You'll also start to crave sugar. L-Glutamine helps with the sugar cravings.
Die-off can be just about any symptom, unfortunately...gas, bloating, stomach rumblings, fatigue, flu-like symptoms, cold symptoms, skin eruptions...I'm sure there's more but that's all I can think of at the moment. They can last for a few days or months off and on, depending on how much healing your body needs to do and what you are doing to facilitate healing.
Anything that causes healing in the body can cause die-off (although not everybody has die-off). So...eating healthier foods, adding coconut oil, kefir/water kefir, kombucha, homemade yogurt, CLO, etc etc.
Ally'smom--fatigue and sugar cravings can be a sign of candida yeast overgrowth.