Free speech is, of course, saying anything you want to without restriction, censorship or consequence. Right now in America you cannot incite violence with your speech, but you can do almost anything else.
Free speech is arguably our most important right, and is guaranteed by the First Amendment. Free speech allows us to defend our other freedoms, and with the free expression of ideas, allows us to grow intellectually and progress. Without the ability to question the status quo, the institution of dogma will result, and authoritarianism will thrive.
Conservatives are not fond of free speech - they believe you can say what you want as long as it agrees with them. Free Speech is particularly frowned upon on Conservapedia , where any edit with a hint of being Liberal is deleted. And unfortunately, "Conservatives are idiotic morons" is not a 'constructive' edit.
See also McCain-Feingold bill.