Construct means to make something by combining materials or thoughts. You can construct a birdhouse using wood and nails, or construct an idea by joining together various ideas or thoughts.
Those two examples help illustrate that a social construct can include values and beliefs that humans have about the construct. Humans can alter the construct as they continue to interact. Attitudes towards those of different skin colors have changed over the last 100 years and they continue to change. The construct of race still exists, but what the construct means has changed.