A matrix means something that has something else regularly embedded within it. For example, a snickers bar has a matrix of caramel and nuts - the nuts are embedded in the caramel. Or we could say that the caramel is impregnated with nuts (these are the actual terms used). Most rocks are matrices because they are mostly one type of mineral, with smaller crystals of another type embedded within them. Note, in mathematics, a matrix is a 2D number grid that has specific rules for doing calculations, but it still means a grid impregnated with numbers.
Attenuating just means "getting thinner", like a wedge. So an attentuating matrix means a lump of stuff, that has something regularly embedded in it, which is thin at one edge but gradually gets thicker to the other.