This acronym expands as "Nearly As Fast As Light".
NAFAL drive ships use the time dilation effect as stated by Einstein's theories: a ship that moves at a speed approaching the speed of light will appear to move outside of time. Those on board will experience that only a short period of time passes by, while those outside of the ship will experience of a far longer period of time.
As an example, a four light year journey feels like ''a few hours'', but in fact it takes four years. The Gum
travels 256 light years in 10 hours 29 minutes. (Vaster Than Empires and More Slow
Obviously, this causes some rather interesting problems: when you go somewhere, and then return to wherever you left from, you may have aged hardly anything, but those who stayed home have aged several years.