While going through reviews and descriptions, you must have come across the abbreviation GSM.
GSM label in fabrics means grams per square metre.
It is the metric measurement of the weight of clothing. 
The higher the GSM, the more solid the fabric will be.
For example, if the label on a cover fabric on bedding material reads 170-200 GSM, the fabric material would be thicker than a fabric with low GSM like 100-120, which is bound to be thinner and flimsy.