The US trade deficit with China is essentially an exchange of goods for financial assets; rather than getting an equal amount of goods in exchange for what they sell to the US, the Chinese are getting financial assets - stocks and bonds, etc. - in return. Or, in other words, the Chinese are purchasing US financial assets with their goods.
Lately, there has been much attention the fact that some of the goods we are importing from China have turned out to be unsafe - toys with lead paint, for example. As Dani Rodrik explains, the recent credit crisis has shown that many of the financial assets that we are selling to the Chinese (and other foreign investors) are also, arguably, unsafe.