Alibre Atom3D is a standalone product that's included in Alibre Workshop. It is a full 3D and 2D CAD system that supports full 3D part and assembly design and basic 2D drawings. In short, it's what you'll use to design things that you want to manufacture in MeshCAM Pro for Alibre, the other package included in Alibre Workshop.
An example of a sketch cutting out heat dissipation fins on a 3D model in Alibre Atom3D.
Alibre Atom3D Information
Alibre Atom3D is a big product and has its own section on the website. Check it out in a new tab below: