"... and a cell, on which B comes after "jumping over" checker A, is free"-what does this lines mean?
Thanks.

For example:
If there is a white checker on the cell
a1 and the black one on the cell b2 then
cell c3 should be empty.

And what about it:
1) White a1
    Black c3
    Empty d4
    (there is some place between White and Black)
2) White a1
    Black a2
    Empty a3
    (White and Black on the Same vertical)