I guess you mean the word written with either the crested ibis and the semicircular loaf (ax + t = axt) or with a sun disc nestled in a valley between mountains (axt), or sometimes a combination of...
They didn't "decode" them; they simply read them, as you do with these words now.
Another answer:
The Ancient Egyptians did not need to decode Hieroglyphics. They used Hieroglyphics as their...
Ancient Egyptians had many symbols. Did you mean their system of writing? That's called hieroglyphics. Did you mean the cross with a loop in the top? That's called an ankh.