"Y" = Yes = Seats available
"WL" = Waitlist = No seats available

If you change to "expert" mode, you'll get to see the actual availability, by fare class, such as:

F3 A1 C9 D3 Y9 B9 M9 H9 Q5 V4 W1 Z0

If you do a search, there is a very good thread somewhere in the UA Forum, posted by ozstamps that lists all of the fare classes and what they mean.