Just to expand a bit, the phrase "economy full" means that the entire coach cabin has at least as many passengers who have purchased seats as it has seats. (So if they have sold 130 coach-class tickets and the cabin has 129 seats, you will see "economy full").

The value "Y0" means that they are not willing to sell any more Y-class tickets, Y being the most expensive fare that books into the economy cabin. You may see "economy full" and Y>0 — perhaps indicating that UA is going to slightly oversell the coach cabin and expect no-shows or expect to have to perform operational upgrades or perhaps deny boarding to someone.

You may also see "economy available" and Y0, indicating that although there are coach seats left, UA is not willing to sell them. This is a rare scenario but might happen on a day where UA expects lots of cancellations and is using its inventory-management system to reserve some slack on its flights.