You buy a coach ticket "Y" and get "UP"graded to either J on a 3-class flight or F on a 2-class flight at the time the reservation is made. They are more expensive then a disounted coach ticket but less than expensive than full unrestricted Y, J or F fares. They have travel/rebooking restrictions with them that will vary by route and the amount of seats available for YUP fares are limited for each flight, so sometimes you may not be able to get them at all.

Generally speaking, they make sense where the price of the discounted coach ticket would otherwise be quite high and where sitting in J or F is a distinct advantage. NY-Hawaii routes are a good example or where a YUP fare would make sense in many cases.

On AA.com you specifiy First Class "with Restrictions" (Business Class "with Restrictions" for 3-class flights) to get them to show if they are available.