In Diablo 2 (classic and LOD) all of the speed calculations in the game are based on the game engine running at 25 "frames" a second. (Don't confuse this with your display frames, which can vary based on lag, graphics board, etc.) So, when you ask "how fast is an attack?" you get that answer back in terms of the number of frames it takes.
With bows and crossbows, there are actually two interesting attack speeds: normal and strafe. When you fire an arrow for any attack except for strafe (e.g., normal attack, ice arrow, immolation arrow), that is a "normal" attack. It takes a certain number of frames before the arrow is on its way. When you strafe, you take a normal number of frames to get the first arrow off, and then a smaller number of frames to fire each subsequent arrow.
The notation that DoubleTrouble developed is simply "normal frames / strafe frames". This lets you see at a glance the important "speed rating" of a bow. For example, if someone says that a double bow with 60 IAS is a "9/2" bow, then you know that it takes 9 frames to put out a normal arrow, and if you are strafing that you'll pump out an arrow every 2 frames until you have shot the number of arrows appropriate for your skill level.