It is true that 'low iron levels' can cause fatigue. The medical term for this is anemia
. It occurs because iron is required for making hemoglobin, which is the main oxygen carrying substance in the blood. If you don't have enough hemoglobin, you don't deliver enough oxygen to your body, which leads to symptoms like fatigue, shortness of breath, trouble exercising, etc.
If you think you might be anemic then you should see your primary care doctor
for a check up. Anemia is most common in women who have heavy menstrual bleeding
. It is less common in men, but it can occur. In addition to blood loss with menstruation, anemia can occur if you do not take in much iron in your diet, if you have certain genetic variants in your hemoglobin molecules, or for other reasons as well. The purpose of the checkup is to, first, confirm that you have anemia (which can be done with simple blood tests), figure out why you have anemia (so that a treatment can be prescribed), and, if you don't have anemia, investigate the other potential medical causes of feeling tired all of the time.
As I said, the first step is seeing your primary care doctor!