It's not made of plastic, if that's what you mean... (just joking around)
Live sand is basically sand - made up of finely crushed corals, sea shells, and other such things that has various bacterias and small organisms living in it. These bacterias help break down ammonia into nitrite, and nitrite into nitrate (along with doing a bunch of other stuff)
So Live sand is real sand (from the ocean, or an established tank) that is "alive" with all kinds of critters living in it..
Hope this helps, and i'm sure you'll get a lot of people giving you more information than i can