What is this? And tell me about what are rows when selected 1st and last bit???
S-box stand for substitution box. For every possible 6 bit input we replace with 4 bit output. For visual convenience the input is broken up into 4 bits and 2 bits. Giving a 4*16 matrix in each cell we have the 4 bit output for the input designated by that row and column. The 4 bits are the original bits and the 2 bits are from the expansion copied from neighboring 4 bit blocks.
We now know the DES cipher was designed to be resilient to differencial cryptanalysis which IBM and the NSA new of a whole 16 years before Eli Biham reinvented the technique and told the world.