well, look at diamonds. they are actually the most plentiful gemston on earth, but because there aren't many on the market, they are one of the most expensive- when they could be as worthless as the gravel in your neighbors driveway.
Same thing, if we print more money, and more people have it, stores will see this and start charging more money for goods and services. if everyone has $5, the price of the $1 candy bar will be raised to a $3 (this is what we hear on the news all the time as "inflation" ) What this all means is I can by less materials with the same amount of money. I used to buy 6 candy bars for $6, but now I can only buy 2. this is buying power, and it decreases. it also means that when I save up $25,000 for my trip to italy I get my money exchanged, I have roughly enough euros(money used in italy) for lunch.
One last example. lets say there is no food in the land. the food in your pantry becomes PRICELESS because you know you only have a little bit. but lets say you live in a military storehouse of food. you don't even care if there's a famine outside, you have enough food to feed an aarmy for 5 years. you don't value it like you would if there was only a little bit
So yes we are in debt, but to other countries. if we just print more money they won't value it the same and for every dollar, they demand $3, or $4, or $5, and we actually sink even deeper into debt.