A non-porous surface is one that is sealed and does not have pores. This is important as those pores provide habitats for pathogens to eat, breed, and generally become a problem. Surfaces like glass, aluminum, and soft silicone are all non-porous. Porous surfaces include cloth, sandstone, unsealed wood, and sponge. 

Example of a porous surface under a scanning electron microscope from "Impedance Behaviour of a Microporous PMMA-Film 'Coated Constant Phase Element based Chemical Sensor"

A non-porous surface can also be thought of as a surface that does not allow the free flow of gases and liquids through it. In this sense. soft silicone is mostly non-porous rather than fully non-porous. Aromatic ring solvents can easily move through silicones. Alcohol can also move through silicone to a lesser extent. Because of this, the chemicals that come in contact with  silicone are not always easily washed off, and what a mold is made from as well as what mold release is used is an important aspect of toy safety.