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).