The longest pre-season in America will end one week from tonight when votes are cast in the Iowa caucuses, closing a year of campaigning that has seen 17 candidates drop out before the balloting began. I’m asked a lot of questions about Iowa — usually about how the caucus works and why it matters so much — because the process is opaque and elevated in importance by virtue of its first-in-the-nation status. So as we count down to the start of the active phase of the nomination process, here is a quick and simple guide to what’s going to transpire on February 2 and why it matters.