Ok it is rather simple when you get it, although getting it can be a little harder. A social construct pretty much is the fact that everybody gets their ideas and beliefs from people or events that impact them. Ok time for examples.
A person will have a good view of health if their parents, teachers and friends all have health-promoting attitudes.
Conversly, someone may have a negative outlook on health if their parents are unhealthy and have little health literacy.
i reckon the best way to think of it is compare a 15yr old in sydney and in a 3rd world country. Their social constructs (brought about by their family, friends, social, economic and political environment) would be extremely different.
Consider - a 15yr old sydney boy would expect to spend his day at school playing sport, in a 3rd world country, working and gathering food