what is contract trading?

It's basically trading something which you do not physically have. If I enter into an agreement to buy a car from you for $5,000 in two weeks' time, I can go and look for another buyer willing to pay $6,000 and sell the contract to him then keep the difference of $1,000 as my profit. So, I profited off your car without taking ownership of it. This is the same model used by online brokers when people trade CFDs.