What do marginal rates mean?

Greg Mankiw - Higher Taxes Mean I’ll Work Less

Reasonable people can disagree about whether and how much the government should redistribute income. And, to be sure, the looming budget deficits require hard choices about spending and taxes. But don’t let anyone fool you into thinking that when the government taxes the rich, only the rich bear the burden.

Excellent piece here, read it all. The only mystery here is, how did this end up in the New York Times?