What is the meaning of gender as a social construct?

When we say that gender is a "social construct," we mean that gender is not a result of natural, physical, or biological differences. Those sorts of differences are referred to as "sex" differences: for instance, chromosomal differences, genital differences, and so on. Something like the idea that boys like blue and girls like pink is a gender difference; there is no natural or biological reason for it - it's simply socially constructed, a concept that we all agree with for no other reason than that we agree upon it.

Basically, when someone claims that something is a social construct, they are saying that there is no natural or scientific basis for it; it exists only due to human behavior, thought, and interference.