A confirmed reservation is simply a reservation you have made that has been agreed by the airline, probably in the for an email informing you that your booking has been made and accepted.

A confirmed reservation is a reservation that is confirmed by the airline. When you reserve a ticket, the airline sends you a confirmation that the reservation is valid and final. But in this case that you mentioned, although the airline confirmed the reservation to the buyer, they couldn't accommodate them because they didn't have any free seats. In other words they sold more tickets than they had seats or "overbooked" the plane.

If you show up late after the plane is already boarded (airline speak for everyone is on the airplane) they can deny you to get on the flight if they have sold more tickets than there are seats.(oversold)This can happen even if you have a "confirmed reservation"! If they are oversold on a flight they will offer the people on board money and replacement flights to get off of the airplane. Your best bet if you HAVE to get on that flight is show up early.