I'm 5'7 and weigh 100 pounds. Is this a serious thing? Am I very underweight? I eat moderately but it's hard for me to gain weight.

Underweight. Yes, being underweight can lead to serious health problems. Please see your physician to find out why you cannot gain weight. Stool studies may show parasites, fat malabsorption, blood tests can show gluten enteropathy, for example.
Underweight. I would consider you underweight by 25-35 pounds,but it would be in your best interest to consult a registered dietician who will help you calculate your ideal weight and devise a special diet and possible supplements for you. This should only take one session. I assume that you are otherwise in good health and have had your thyroid,blood sugar,etc checked.