Shadecloth is measured in various ways. One way is GSM- or grams per square metre. Typically the higher the GSM, the heavier and higher the UV blockage the cloth gives. So for instance, a shadecloth with a 185gsm would be considered quite light weight and would only give a relatively low UV blockage whereas a cloth with a 280gsm is considered quite heavy and would block out a much greater amount of UV radiation.
BEWARE- if the GSM is not mentioned when you purchase a shadesail, you should assume it is a low GSM, therefore lesser quality.