You should only be getting this screen if you are doing an amended return. Basically, an explanation statement is simply the reason that you are amending your return. So, if you got another W2 after you filed and you have to amend, then you would put something like, "I am amending my original return because I received an additional W2 after I filed." They want a statement so that they understand quickly why you are amending without having to look at the whole return.