What is the purpose of s-box in DES?

The S-boxes are the nonlinear part of DES that makes it difficult to break the algorithm and secure against linear and differential cryptanalysis. the s-boxes provide the "confusion" of data and key values, whilst the permutation P then spreads this as widely as possible, so each S-box output affects as many S-box inputs in the next round as possible, giving "diffusion".

There are 8 s-boxes also known as the substitution boxes, is a table that consist of four rows and 16 columns with 64 entries all together. They take in 6-bits and produce or output 4-bits. That is, the 48-bits into 8 S-boxes will be 6-bits each. However the 6-bits is represented in binary form of say, 010100. The two outer bits (the first and the last bit) represents the row (one of the four rows) and the inner four bits represent the columns (one of the 16 columns). The cell where the row and the column meets represents the value in decimal of the output. This is then converted to binary as the output. From the example 010100, the first and last digits 00 = the row which is the first row (00, 01, 10, 11) and the inner four digits 1010= the column. All 8 S-boxes will output 4-bits each in similar way and that is 32-bits output that is then permutated and further processed in the next round.