As someone who runs an software outsourcing company, let me explain it as I understand it.
The answer is that every time the USA needs to finance debt, they sell treasuries at auctions. In the old days, these were bought by domestic buyers, but now-a-days, it's mostly foreign buyers - specifically, countries that have a lot of USD and nothing to do with them, China, Japan, and oil-producing countries.
The Chinese do not float their currency, it is controlled by their government. It is artificially low against the USD (though they are correcting that slowly), and this has encouraged many people to move factories and resources off-shore to China.
This has allowed China to build up significant skills and factories. But, because they are paid in US dollars, it has created inflation. The first thing that companies that sell to the USA do is take their dollars to the bank and exchange them for Chinese yuan (also called the RMB, translated literally, "the people's currency").
So, there is China, with billions and billions in US dollars and no idea what to do with them. Well, they use a significant part of them to buy US treasuries, effectively buying US debt. On the surface, this seems like a great idea - they finance the US debt, and Americans keep buying their cheap crap.
The problem for the US is the debt is not sustainable, and it is causing our factories (and high-tech) jobs to go to China, leaving us with a "service" economy that is quite hollow. The problem for China is they are buying US debt which will probably never be paid back, and they are holding USD reserves that are worth less every day.
If the Chinese ever try to use significant amounts of their reserves, or dump them on the market, it would create significant inflation (these dollars were just printed from the printing press, and are out of circulation as long as China doesn't use them).