Boring is a term that means that the instrument is cutting on the tip and usually means that there is a screw that pulls the instrument into the material.
Thus is mostly a term that is used with tools that are designed for cutting wood or soft plastic.
However, in the sense that there are really no endodontic instruments that do that, we can use the term to mean any instrument that has end cutting capability.
Most K-files are of this type.
The implication is usually that the bottom of the hole that they bore is the same shape as the tip of the instrument.
If that internal tip angle is flat it will produce as shape that is usually referred to as a ledge.
Some instruments have sought to avoid some of these problems by decreasing that angle.
One that immediately comes to mind is the Flex-R file, designed by Dr. Jim Roan at the University of Oklahoma, which has an included tip angle of 59 degrees instead of the usualy 90-105 degrees of a K-file.
This produces a shape at the apical extent of the prep that allows instruments to slide past it into the canal beyond the prep.