Number of Shells refers to the thickness of the exposed outermost layers of the printer model. 2 shells is the normal. Increasing the number of shells creates thicker and stronger walls, but also longer print times.
Infill Density defines the amount of internal support structure inside the 3D print. Generally, the higher the fill density, the stronger the object but also the longer the print time. Standard print infill is 20% and you should never go above 60% or 0% which will make your object hollow.
Sometimes, 3D models have "overhangs" - layers that extend outwards that are usually unsupported over previous layers.
Solution:
Quality and Layer Height refers to print resolution or print quality and changes the surface quality of a 3D print. There are three options:
The general rule of thumb is: the smaller the layer height, the higher the resolution/quality, but the longer the print time.
For spools that don't fit into the filament drawer, please use the new red filament spool holder located at the back of the MakerBot (see below).