I got this answer in another post, just adding here for those searching to find out what these mean:
"The number refers to the number of sides on the dice. So a 1d4 has a 1,2,3 and a 4 side so the highest number it can roll is a 4. A short sword has 6 sides 1,2,3,4,5, and 6 so the highest number you can roll is a 6. The average of a d4 is 2.5 and the average of a d6 is 3.5 so a 1d6 will on average do more damage."
http://forums.larian.com/ubbthreads.php?ubb=showflat&Number=679304#Post679304
