A social construct is a category or thing that was basically created by society. For example, a virgin girl or a non-virgin girl.
Race is a social construct because there are no "unique "black genes" or unique "white genes." Society created race and ethnicity. The only different thing is color of skin and physical features.
Just as there isn't anything unique to a virgin or non-virgin girl, except for their "virginities."
Good luck and I hope I helped this to make sense! :)