Bearing a close relationship to the famous Dimethicone, Methicone is the slightly trimmed down version missing the methyl (-CH3) groups on one side of the silicone chain. Like most silicones, it has nice emollient properties and improves the spreadability of products.
But Methicone's main thing in practice is not being an emollient but a silicone fluid for hydrophobization treatment of powders, i.e. making solid powders (mineral filters & color pigments) very water resistant and easily spreadable. Methicone does this by absorbing traces of water from the surface of pigments that is very useful for mineral sunscreens and makeup products.