The letters represent fare "buckets" and the numbers represent
the number of seats that US Airways is willing to sell at those
fares. The number 4 means "at least 4, maybe more." Fares get less
expensive as the "buckets" move to the right in your display.
(That is, tickets booked in the "K" bucket are less expensive
than tickets booked in "Y" bucket.) These buckets are used
by US Airways to adjust the price of tickets on a certain flight -
they only sell cheap tickets if the flight is not full. As the flight
fills, they typically do not sell seats in the cheaper buckets,
and you are seeing this in your display.

"F" and "A" classes represent first class. F is usually a full-fare
first class and A is discounted. Your line says that US Airways
is selling at least 4 (maybe more) first class seats on your flight,
but they are not available at a discount fare.

The other buckets are coach seats... Y is "full-fare" coach. Your
line says that US Airways is selling at least 4 coach seats on your
flight, but they are not selling them in some of the less expensive
fare classes. ("R" class may be something special on US... I am
not sure.)

The bottom line of what you see is that the flight you are looking
at has some seats for sale, but it is not so empty that US is trying
to sell seats at low prices. If you are looking for a cheap flight,
this one is probably not going to work - you will need to find a
flight that has something like K4 or V4, where "K" and "V" buckets
are cheaper fares.

Hope this helps...