"Pay with miles" means using your miles like cash - buying a ticket and reducing the cash price with a "pay with miles" feature that's shows up somewhere during the booking process. You get a value of 1 cent per mile. Generally, this doesn't get you the best value. Generally, the preferred way to approach it is to look at the price in miles - this gives a miles price of such-and-such number of miles per ticket. Also select flexible dates - this gives a date range with miles amounts for the different dates and there can be huge differences.