A lefse rolling pin is a rolling pin with 3 mm (1/8 inch) wide grooves carved into the surface. It is designed specially for making the Norwegian flat breads known as lefse.
Single-cut ones have grooves that run around the circumference of the pin; double-cut ones have grooves that run the length of it as well, thus where the lines intersect forming knobs that are about 3 mm (1/3 inch) square.
The grooves help press air out of the dough, while at the same time making it harder for the dough to stick to the rolling pin. When you flour the rolling pin, the grooves allow the pin to pick up flour, making it more non-stick.
The whole rolling pin itself is about 25 cm long (10 inches), and about 7 1/2 cm (3 inches) in diameter, with 9 cm (3 1/2 inch) handles on either end.