This file defines the modification of weapon speed, damages inflicted and to hit bonus, based on weapon proficiencies.
First column simply is the number of stars in the given proficiency
The columns 2-4 are modifications to the features described by their names (to hit bonus, damage bonus , speed bonus)

In BG2 ToB we have:

2DA V1.0
0
    HIT   DAMAGE  SPEED
0   0     0       0
1   0     0       0
2   1     2       0
3   2     2       0
4   2     3       -1
5   2     4       -3