The weight of a towel is measured by the GSM (Grams per Square Metre). Low GSM (300-400) towels are light and thin whilst high GSM (450-600) towels are thicker and heavier.
A towel that weighs less than 400 GSM is likely to be thin and typically suitable for a gym bag or trips to the beach.
If you are purchasing a hand towel or guest towel then it is advisable for the GSM to be at least 400.
For Bath Towels, a higher GSM of 500 will provide a soft and absorbent towel. Typically, the higher the GSM, the better performance you will have from the towel.