It's the engine displacement. It's basically the bore (diameter of the cylinder) times the stroke (how far the piston travels in the bore) times the number of cylinders. So, the LS2 has a 4.0" bore, a 3.62" stroke and 8 cylinders. So, the formula for displacement is bore * 3.14159 (which is the area of a circle) * 3.62 *8. For those that don't recognize it, the 3.14159 is PI. So, you get 12.56636 *3.62 * 8 = 363.9217 cubic inches. To convert cubic inches into Liters, you multiply 363.9217 by 16.387 (that's the conversion factor for CI to CC) and you get about 5961 CC, and a 1000 CC is a Liter, so it's 5.961 Liter, which I guess is close enough to 6.0 Liter.
Anyhow, that's more math than anyone should have to read about after 3 beers. And I may have rounded a few values, so I'm sorry if I'm off by a few CC's...