Deficit: flow. Debt: stock. It is meaningful, because even if you can service it forever, the cost of servicing it could have been spent elsewhere, forever. The present value of all future debt servicing payments, taking into account the government's cost of capital, is... $18T. If we didn't have $18T in this debt, we could have $18T in some other debt -- we could be financing $18T worth of something else. I don't know how it's not meaningful.
I am no expert, but I find it better to look at debts credits together. Debt looks scary, but when other countries owe the US trillions too, it balances out somewhat. The net international investment position is an aggregate measure of how much over-leveraged the country is as a whole, and looks much less stark. This combines government debt with citizen assets, which is good and bad - masks federal policy issues, but also accounts for the artificially low taxes in the US and the impact that has on relative wealth placement.
I'm curious as I've only ever seen "<country> has £<bignum> sovereign debt" and the other side of the equation is never really mentioned. But does the value of what the US has lent other countries approach $18T? Seems like it'd be hard to find even a handful of large enough countries you'd want to spread that sort of risk over.
In general, the foreign debt taken on by the US is similar - e.g. bonds issued by various countries, and bought by the citizens and the government as an investment asset.
