Besides adding up to 4, 2+2 is a designation for automobiles which have a minimalistic back seat, which is usually fairly useless for anyone who is not a child, earning nicknames like the "Japanese torture trap".
2+2 seating configurations are employed in order to increase the sales numbers of some cars, primarily lower-end sports cars. This works for two reasons: First, if you are willing to squeeze, it is possible to fit four adults into such a car as long as at least two of them are fairly diminutive. Children, of course, generally have little trouble fitting into the back seat of such cars, getting in and out, and so on. Most people hesitate strongly before buying a two-seat car. The second reason is that cars with only two seats are in a different class as far as insurance is concerned. While the cost for liability-only insurance is more or less unaffected by the nature of your car, at least in the US, higher levels of insurance will cost more for a two seater than the same car would if it were a 2+2 seater, even if the back seat is fairly useless.
Note that this is 2+2, not 2+3. This is primarily because most cars which have this configuration are sports cars, which means rear wheel drive, floor pans well below the rocker panels, and with a high tunnel for the driveline (and to stiffen the car.) This means that the tunnel generally is also very high under the back seat, and the center of the back seat is purely cosmetic, typically consisting of less than an inch of foam cushion covered in upholstery.