By "hit dice", what you're referring to in this instance is the hit points of your character rather than a damage roll.
You may well be aware that Advanced Dungeons and Dragons uses weird looking dice that look like this;
As MM-Rogovich indicates, 2D4 translates to 'two rolls of a four-sided die' (a four-sided die looks similar to a pyramid - in the image above it's the black one on the end) - at worst the rolls of two four-sided dice will be two, at best eight.
Fighters use a ten-sided die for their hit point rolls (as I'm sure you know, hit points are how much damage the character can endure before 'shuffling off this mortal coil'), so therefore in the example 9+3 translates to 'nine rolls of a ten sided die plus three points'. The character's Constitution score will also affect a character's hit points.
According to the manual, Mages and Sorcerers use a four-sided die for hit points, Thieves and Bards a six-sided die, Clerics, Druids and Monks an eight-sided die (albeit it says ten-sided in the SoA manual), and warriors a ten-sided die. Note that although the ToB manual says Barbarians use a ten-sided die, the game itself indicates a twelve-sided die roll.
One guide you might find of use is Dan Simpson's explanation of AD&D rules, being the last entry
on this page at Sorcerer's Place.