It's a bit hard to answer your question as I'm not sure what you're looking for.
If you are asking what reduces the tranmission of light through glass, the most basic answer would be thickness, composition, and texture.
If you are asking about soda lime float glass specifically (i.e. window glass), most of the tranmission losses in the visible spectrum come from reflection at the surfaces (~8.5%) and absorption of light by iron in the glass. Hence high transmission float glass is called low iron glass. Anti-reflective coatings on the glass surfaces can reduce the reflection losses (as can some textures).
