what does this number(spacing between pads) in the datasheet mean?
1.27 refers to the pad-to-pad pitch. There are 24 pads per column, so 23 intervals. 23 times 1.27 is 29.21 which is shown below. The "2-" refers to the fact that this spacing occurs in two different places, once on the left column and once on the right. It lets you know that the specification does not apply only to the column shown. 

Likewise, the specification for the horizontal row small pad spacing "2-1.27(15x)" shows that 16 pads are involved, and the spacing applies both to the upper and lower rows.