Positioning precision, what is it and how does it correlate with the CraftBot’s performance?

This is a measure of the accuracy in which the extruder moves along the X,Y axes. It is not the layer height (which the CraftBot is able to print at 50 microns/layer when a .25mm nozzle is used). This Positioning precision value however, can be as low as a 4 micron accuracy!    

In a few sentences, what is 3D printing?

While regular 2D printers only print in X,Y dimensions (and usually deal in only texts), 3D printers allow you to create physical objects by using melted materials, quite often in the form of a filament; they build these objects layer-by-layer. In the end the only limitation is your imagination!