Well, it unfortunately is not a simple idea to explain!
The simplest way to explain it is that it is the relative density of the water. Fresh water is less dense than saltwater, because it has less "stuff" in it. When you add salt to water, it increases the amount of salt concentration in the water, making the water more dense.
I found this article on Wikipedia, but it may do more to confuse you than answer your question...it's not extremely easy to follow.
Anyway, I hope that helps a little...maybe someone else can explain it better!
38 Gallon (Freshwater): 1 Green Swordtail, 3 Yo-Yo Loaches, 1 Clown Pleco, 1 Blue Gourami, 4 Otocinclus, 7 Cherry Barbs.
46 Gallon (Saltwater): 60 pounds Lalo Live Rock, 60 pounds aragonite sand, 1 Coral Beauty Angelfish, 1 Peppermint Shrimp, 1 Mushroom Coral, 1 Emerald Crab, various snails and hermit crabs.
10 Gallon (Saltwater quarantine):