It means the ribs are capped with 1/8" thick x 1/4" wide strips of balsa.

The rib is capped with 'cap strips'. these are balsa peices, as stated above, and glued to botht he top and bottom of the exposed rib, creating an 'I' beam in effect, and adds structure to teh wing. The thickness of teh cap strip is usually always teh same as the wing sheeting. They butt up against the LE sheeting,and teh TE sheeting..