It goes along the lines of:
China imports around $50billion worth of US goods
US imports around $300billion worth of Chinese goods
Therefore: US has a trade deficit with China of around $250billion dollars. Any friction between the two countries could jeopardise that trade, leading to possible economic problems in the US.
At least that's how I understand it - I'm keen to hear if I'm along the right lines or if I've got the wrong end of the stick.