21 in base 2 is
010101
The column values are:

or

21 = 16 + 4 + 1

Each card contains all the numbers that have a 1 in its binary form in the column whose value is given in the top LH corner of the card.

So the card that has 4 in the top LH corner contains all the numbers that have a 1 in the 22 position.