Wicks basically means that the wetness from the inside of the pocket starts to seep through the outer layer and makes the outside of the nappy and any clothes over it damp. It is common around the leg holes.
Some pockets are worse for this than others, pockets with a cotton outer are the worst. Pul outers tend to be the most reliable.
If I described a nappy as leaking I would mean that the fit is poor and that it is not containing things properly or that there was defect in the material or stitching that causes it to leak.