Molding Question:What is this?

I´ve seen this "cutted rolls"on some pics from molds.What is it?
I know that its to hold the siliconmold in shape..but I want to buy this material.

 

That is silicone cast in a tube. It is then cut into lengths to make keys in the mold.You make them yourself from the silicone that you use to make your mold. You can also pour into a box made of foamcore/sheet plastic and make squares and long keys the same way.

 

As user HMS Mike said, they act as keys for the mold. Usually used in conjunction with a mold casing made from a harder material. An alternative method for making these is simply to chuck some silicone into an old ice cube tray, which will give you nice uniform blocks for future mold keying!!

 

To make keys, I just let any extra silicone left from earlier pours solidify in the bucket. Then I cut it in to strips or other shapes to use as keys.