It's pretty clear that a government that doesn't repay its bonds is in a state of default, a state of explicit default. A state where it just says "we can't pay you back, you'll have to take 50 cents on the dollar" or whatever it turns out to be.
When a government is in control of its own currency it can create the money it needs to repay its bonds if needs be, and so the possibility of an explicit default is taken away. If the US needs $1trn to pay some bonds it can pretty much create that trillion out of the air and pay the bonds. But this is really a default in all but name, hence the term "implicit default". The issuer can repay the bonds but only by devaluing its currency.
To put what that means into something that resounds with the average person more than numbers with lots of zeroes on the end, let's say a pizza costs $20 but you invest that $20 in a government bond paying 5% per year. Inflation is running at 5% so even though next year you get $21 back from your bond, it costs you $21 to buy a pizza. So effectively what happened is that you invested a pizza, and you got back a pizza.
Now let's see what happens if the government has to repay your $20 but only has $10. So it prints another crisp $10 bill to repay you. All else being equal the price of things increases, as there are more dollars chasing them, so your pizza now costs $30. So you invested $20, you got back your $21 (including interest) but a pizza now costs $30. In other words you invested a pizza and got back 2/3 of a pizza.
If a government goes into explicit default it becomes much more expensive to borrow, as lenders want a risk premium built into the price at which they lend. If a government goes into implicit default then sooner or later the same happens, as lenders realise the money they get back at the end has less buying power than they expected and demand a higher rate to compensate.
I find it mind boggling that anyone is willing to lend to the US government for 10 years at a rate that's comparable to the official inflation figures.