It’s a bit complicated.
You have two different sets of numbers. The first set appears in the schedule. For example, on this LAX-JFK search:
0471 LAX JFK 21Sep 11:35AM 21Sep 08:05PM DL 767 0/0(26) 0/0(0) 13/13(200)
There are three types of numbers. The first, 0/0(26) is business class available seats (0), authorized (0), and capacity (26). The second is the same for first class, the third is the same for economy. What do these numbers mean? Let’s focus on the economy numbers. There are 13 seats available (i.e. unsold), 13 authorized for sale (sometimes more seats are authorized in anticipation of no-shows), and 200 total seats in economy. Sometimes you’ll see negative numbers, which implies that they’ve overbooked the flight.
Now, if you click the icon at the end of the row (it says “standby list”), you will get to see the airport lists. The first thing you see is the standby list, and you can see your own position if you’ve already booked. Then, click “view airport standby list” to see more. In this flight’s case, here’s what was shown:
REV = 6
NONREV = 4
Seats Not Assigned:
FIRST/BUSINESS = 0
MAIN CABIN = 15
This flight is leaving in about 30 minutes. So there are still standbys on the list that have not gotten seats. It looks like there are zero first/business seats left, 15 economy seats, and 10 standbys. So, it looks like everyone will get on that flight.
Note that this information is useful just prior to departure. But if you try to look at a flight that is days or weeks out, you will see a lot of available seats in the airport standby list because no one has checked in. Once people start showing up at the airport, those numbers start to move.
Sometimes, especially at hub cities like MSP, things can go totally opposite of what you expect. For example, a connecting flight might be late, and many of the listed standbys don’t make it to the flight, and you somehow get on.
On the other hand, a flight might get cancelled and all the passengers from that flight might get moved onto your flight, pushing you off.
Bottom line is that you can never be sure what will happen. Have a good plan and backup plan in case you don’t make the flights you want. The numbers are hard to really trust, except for when there are lots of open seats. When you see large numbers of seats (like more than 25 or 30% of the capacity), you usually have a pretty good shot.
Good luck, hope this helps!