The first number is available seats (number of seats on aircraft - number of seats sold). The second number is authorized seats (number of tickets to be sold on flight - number already sold). 2nd number doesn't mean too much really. When looking at your chances to get on a flight, compare the number available to the number of standbys listed.
At flight time, at the gate, they will continue filling the aircraft until it is either full or overweight, whichever occurs first. Just because a flight might be oversold (negative seats available) doesn't mean you won't get on. There are misconnects, no shows, cancellations, etc. I've seen a flight at -25 depart with open seats.