A workplane is a point in space specified by coordinates and consists of three orthogornal vectors. They are used as local coordinate systems or reference points for entites such as toolpaths. They are also used for transforming entities.
Workplanes are helpful as they enable you to correctly position entities and setup meaningful directions when calculating toolpaths.
You can access many of the functions you need to work with workplanes from the Workplane tab on the ribbon.