The 18 month financing for MacBook pro, how does it work??

Contat the AppleStore and ask.

In the past, they have allowed Barclays to sign you up for a credit card with a limit big enough to charge your Mac equipment. If you pay the entire amount by the due date, there has been no Interest, BUT if you miss paying it off completely, you will be charged huge amounts of Interest on your monthly balances going back to day one.

If you take this option, make sure you pay off at least 1/17th of the total every month, without fail, to be certain to beat the deadline and not get clobbered with Interest in excess of 10 percent per month. If you charge anything else on the card, pay those off monthly, as those other charges will not be "no Interest".