It means that is how much the material is going to shrink, per cm or per inch, you will have to add this to your model, normally done as a percentage
If you have a cavity that is say 4cm wide then you will add the material shrinkage number to that, but shrinkage will depend on your parts shape, the thickness of the part Etc, these numbers are just a starting guide, if you need a precise size, then you will have to run the part, then measure the part, and remachine the cavity to get the size you want, if your part size is not important, then just use the shrinkage they suggest and you will be close to the size you want