"Worming" the line is designed to fill in the channels (the contlines) between the strands in order to keep water out and to allow tighter, smoother wrapping of the next layers by giving the rope a more cylindrical shape.
Lengths of "small stuff" or string are led along the lay of the rope between the strands, following the twist so that they spiral round the main line.
This is wound from bottom to top, the edge of the progressing strip slightly overlapping the previous wrap to create a shingled effect, to prevent water from entering.
This needs renewing periodically, and going aloft to paint footropes, shrouds, stays, and other served rigging is one of the regular maintenance tasks on many tall ships.
Many "recipes" for slush exist, but the intent is always to allow a penetrating coat of preservative pine tar that then cures to a harder finish that will not so easily rub off on sails and crew.