Race, at least as popularly imagined, is a cultural construct. People have different levels of skin pigmentation, which is a biological fact. But virtually everything else that we associate with race is a cultural construct. Some have argued that disability is a cultural construct. Some people have difficulty with mobility, but this is a problem because the architecture and logistics of our society are privileged toward mobility. It is a major debate in education- there are certain disorders (ADD, for example) that become serious because we structure education to privilege people who can sit still and pay attention for extended periods of time.
A social construction is something that we make up as a society. This is as opposed to some sort of biological fact. For example, sex is not a social construct. By your biology, you are either male or female. But gender is a social construct. It is what we expect males and females to be like and that is based on our societal values and prejudices.