Wicking is where the individual glass fibres behave like straws and absorb water along their length, so they swell in size and are easily identified by a slight relief of the original fibre visible along the gel coat.
Very often when the water dries, the swelling subsides and the pattern disappears. When looking at a gel coat without pigment, the wick is easily identified because the affected area will have many individual glass fibre filaments clearly visible with a white outline.
This white outline is the point at which the bond has broken between the resin and each individual filament. This "absorption" indicates that there is moisture in the resin and is often a precursor to or accompanies blistering.