This file defines racial 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 races identifiers.
The rest columns are filled by modifiers for given race - pretty obvious from names I suppose.
In BG1 TotS we have:

2DA V1.0
0
           PICK_POCKETS OPEN_LOCKS FIND_TRAPS STEALTH
HUMAN      15           10         5          10
DWARF      15           20         20         10
ELF        20           5          5          20
GNOME      15           15         15         15
HALF_ELF   25           10         5          15
HALFLING   20           15         10         25