These digits stand for the numbers of ways they can join other digits to form a domino tile. 2 is for digit being in the corner of the playing field. 3 is for digit touching the border line of the field and 4 for digit in the middle of other digits. Look at the pictures below.
2…3…4 situation is for initial (“empty“) board. Every discovered tile modify this for every digit that touches newly discovered tile.
𝐃𝐎𝐌𝐈𝐍𝐎𝐒𝐀 