We don't have enough experience with large numbers to hold them in our minds. A million, a billion, and a trillion are all just abstractions for "many" that we tend to equate with "more than I would care to count."
But they are hugely different!
Here's a simple illustration: count to ten. That's roughly ten seconds and easy to grasp. Have you got that length of time in your head? Okay now, how much is a million seconds?
One million seconds = 11.57 days, or more than a week and a half. That's a lot of seconds! A million seconds might measure the amount of time between laundry loads or of a pretty good vacation.
One billion is one thousand millions.
One billion seconds = 31.69 years. That's a big chunk of a person's lifetime. A billion seconds ago it was 1982 and John Belushi died. Michael Jackson released Thriller. Madonna's musical career began in earnest.
One trillion is one thousand billions or one million millions.
One trillion seconds = 31.69 millennia. One trillion seconds ago it was about 30,000 B.C. and earth was in the Pleistocene epoch. Glaciers covered 30% of earth's surface. Mammoths, mastadons, saber-toothed cats, and Neanderthals were about to become extinct.
Compare a laundry cycle to Madonna's career. Compare both of those to the time of the last ice age.
The current national debt in America is over $17 trillion dollars (usdebtclock.org).
In 2012 PBS received $27 million from Congress.
I have heard some conservatives claim that cutting PBS's funding will restore balance to the debt (or at least reduce the burden somewhat, "sorry, Big Bird"). You'd need to cut half a million PBS's before you'd start really noticing any change to the debt.