The S-Boxes are the most important part of the DES algorithm. They are
4x16 tables, which are used to map 6-Bit binary numbers to 4-Bit binary
numbers. The first and last bit are used to determine the row, the middle
4 bits are used to determine the column. The value in the table is the
output.
If we, for example, have the binary number 100110, we would look at the
first and last bit, which are 1 and 0. This would give us the row 2. The
middle 4 bits are 0011, which would give us the column 3. The value in the
table at row 2 and column 3 is then our output.