I found this kind of fascinating...when I signed onto AIM the daily news and stuff came up. and something that caught my eye was Egyptian Funeral Spells Found...and so i clicked on the video. These archealogists were digging in Egypt and found the Queens (her name escapes me at the moment) tomb and near the tomb were Heiroglyphics that they said were Funeral Spells to help the Queen move on safely to the afterlife....thought id just tell you guys. lol
Well, there is a thing called the roseta stone, that has the writing of three languages in one place, all saying the same thing(or very close to it). Kind of helps for translations a bit.
As to the meaning of some other ones, it kind of goes on logic. Meaning that once the heirglyphics are translated into words, there meaning is then put together. So a collection of words like that in the Book of the Dead, leads one to believe it was a book for the afterlife(same goes with writings on the wall).
Arguably we can't be certain that we are correct with the Rosetta stone however considering that it allows a viable translation of every hieroglyphic source we've encountered I think it's probably reasonably certain by now :)