With the Thicken tool you can quickly add dimension to flat geometry. (This functionality is often referred to as Shell.) You can use the Thicken tool to add thickness to the outside of a volume to make the object larger or to the inside of the volume to create an inner surface.
When you invoke the Thicken tool, it creates a new set of polygons at the same location as the original polygons.
Offset: Moves the selected or original polygons in the direction of their normals (with a positive offset). At the same time, Modo creates a set of polygons in the original location but facing the opposite direction. Modo connects these two sets of polygons with a row of polygons that are roughly perpendicular. If the offset is negative, Modo pushes the original polygons in the opposite direction of their original normals and flips them at the same time to avoid having the object turn inside out.
Thicken also has a Scale option.