Complex sentence: The difference between an orbital and a sub-orbital spaceflight lies in the fact that an orbital spaceflight means the spacecraft attains sufficient height and especially speed to travel in space around the Earth at sufficient speed for the centrifugal force to cancel out the pull of Earth's gravity.
Split into simpler sentences: The difference between an orbital and a sub-orbital spaceflight lies in the fact that an orbital spaceflight means the spacecraft attains sufficient height and especially speed to enter into an orbital trajectory around Earth. Travelling on an orbital trajectory in our case means moving around Earth at sufficient speed for the centrifugal force to cancel out the pull of Earth's gravity.