This tool is typically used after the Plant Guides tool, as it interpolates guides between the ones you already have. If you put down a handful of guides and groom them, you can use this tool to quickly and easily fill in the rest. This is useful to quickly get a rough shape for the fur on your character. It is also typically used for fur, or characters that are completely covered in hair.
Using Reguide with Plant Guides
Click the Plant Guides tool on the Grooming tab.
Place the guides by clicking on the skin of the character.
Groom the guides to the rough shape you want the fur using the styling tools.
Click the Reguide tool on the Grooming tab to fill in the rest of the guides, with the same length and shape of the existing guides.
Scrolling the mouse wheel or holding ⇧ Shift while dragging will let you resize your brush.
For specific parameter information, see the Reguide SOP help.