When applying water to paper it moves and buckles (known as cockling by papermakers) because the application of the wash to the sheet causes one side to expand slightly.
However, the other side remains dry, and due to the sizing contained within the sheet, it does not expand. To counteract this expansion on the wetted side, the paper does the only thing it can, and bows and buckles. This is very unsightly for the finished painting, and difficult to work with, because the paint will pool in the waves of the paper.
Bucking mainly affects paper weighing 300g/m² and below.
To stop the paper buckling in the first instance, soak and stretch the paper prior to use, use a heavy weight paper that doesn't move much, or use a block glued on all 4 edges.