Concrete Wire Mesh Video
What you see is a piece of wire mesh that is 6 inches x 6 inches. It is what is called 21 pound mesh. It is pretty light weigh and the best part about this particular mesh is it comes in flat sheets. Typically, it is purchased in rolls, but I don't recommend it as it is very hard to unroll it and get it to lay flat.
What the concrete men can do it tie it together if they want. But as the concrete is being poured, they will pull up the mesh and get it in the center of the concrete slab. The importance of the steel mesh is concrete has great compressive strength, meaning you can squeeze it and it won't fail. But it only has one-tenth the strength when you pull it apart or try to bend it, as compared to compressing it. But steel on the other hand has tremendous tensile strength, so it takes a lot to pull the steel apart.
So putting steel in concrete, you make it much, much stronger. It's that simple. Make sure you have reinforcing steel in your concrete.