What is "rigging"?

Nope, rigging has absolutely nothing to do with models for 3d printing. You can worry no more ;]

Rigging is when you connect a 3D mesh to a skeletal armature that can be articulated. It is important for posing models such as characters, either for static renders or animations.

But it is not needed (or even advantageous) for a 3D print model.

So, as limonadinis said, worry no more! :)

Terrific! Thanks, folks!

No need. It this the phase before the animation.