When American imports goods from China, there's a demand for Yuans and a supply of USD. What is someone in China supposed to do with a USD? They have two options. 1) They can buy a US export. 2) They can buy US stocks and bonds.
If the Chinese buy US stocks and bonds, the transaction acts as a loan that finances US consumption with the promise to pay back at a future date. This cartoon suggests that the debt is never repaid.
As long as the US is able to make for GDP than it owes, this behavior will likely continue. As a cautionary note, however, each import is a claim on US assets.