To polarize is to divide. Something that's been polarized has been split into two sides that are so different, it seems as though they're from opposite ends of the earth — like the North Pole and the South Pole.
Political parties have long been polarized by different views and approaches to government. But you'll also find polarized soda devotees arguing over the merits of Coke vs. Pepsi. Any topic that is controversial can polarize a population, especially if the topic inspires either a fiercely "pro" or fiercely "con" reaction. Topics such as gun control and health care reform have been known to polarize Congress.