This file defines dexterity bonus or penalties to thieving skills.
The first row (titles of columns ) is filled by the names of features affecting of PC (like PICK_POCKETS for example).
The first column is filled by levels of dexterity.
The rest columns are filled by values for given level of dexterity - pretty obvious from names I suppose.
In BG1 TotS we have:

2DA V1.0
0
           PICK_POCKETS OPEN_LOCKS FIND_TRAPS STEALTH
9          -15          -10        -10        -20
10         -10          -5         -10        -15
11         -5           0          -5         -10
12         0            0          0          -5
13         0            0          0          0
14         0            0          0          0
15         0            0          0          0
16         0            5          0          0
17         5            10         0          5
18         10           15         5          10
19         15           20         10         15
20         20           25         15         18
21         25           30         20         20
22         30           35         25         23
23         35           40         30         25
24         40           45         35         30
25         45           50         40         35