This file indicates base thief skill points, and racial bonus or penalties.
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.
In BG2 ToB we have:

2DA V1.0
0
            PICK_POCKETS    OPEN_LOCKS      FIND_TRAPS      MOVE_SILENTLY   HIDE_IN_SHADOWS   DETECT_ILLUSION   SET_TRAPS
HUMAN       15              10              5               10              5                 0                 0
DWARF       15              20              20              10              5                 5                 10
ELF         20              5               5               20              15                0                 0
GNOME       15              15              15              15              10                10                5
HALF_ELF    25              10              5               15              10                0                 0
HALFLING    20              15              10              25              20                0                 0
HALFORC     15              10              5               10              5                 0                 0