Is the national debt/federal deficit important or not? Why or why not?

I keep hearing conflicting things about the deficit and debt. I hear that we need to be fiscally responsible and stop so much spending because the national debt/deficit is bad. At the same time, however, I hear that the deficit isn't that bad if we're spending it on the right things like medicare and education. Well which is it?! Is the deficit/debt bad or not?!