DES S-Box properties
First property says- 
If you consider a 6-bit Inpput Difference(Difference means XOR in case of DES).i.e  6-bit binary number which is the xor of two inputs(obviously both 6 bits).When you enter both of these inputs to Sbox and get two outputs each of 4 bits and xor the outputs what you get is called Output Difference.

Total 64 inputs are possible hence 32 pairs having a same difference.

So this property says for any  non-zero 6-bit input difference there can be maximum 8 possible different 4 bit output difference and not more than that.

Hope i am clear