Tradies, is slang for tradesmen. Tradesmen are those who work in the trades, and do not have a good social reputation compared to white collar workers. This is because their line of work usually require skilled manual labor. Those are fall into such line of work are usually more brash and physical compared to white collar workers who sit in the office all day.
Regardless of any social differences, it is undeniable that tradies on average have more physicality than sedentary white collar workers purely because of their line of work. Working manual labor for 8 hour shifts is very taxing on the body, and some even work out in gyms after that.