How Serious Is Our National Debt?

Okay folks, this is an issue that's been bothering me for a while, mostly because my right-leaning brain instantly shuts off as soon as someone mentions math. But if some folks here have some extra time and would like to explain this to me in layman's terms I would be eternally grateful.

I know that our national debt is somewhere in the trillions, and a big chunk of that is being borrowed from other countries. So what does this mean for us? Are we really beholden to China like some say? Or are the Chinese more beholden to us because of the trade imbalance? What other things are causing this debt and where is all the money coming from exactly?

And how big a problem is this in reality?