basically a thing that screws onto the top blade of your calipers and allows you to get a measurement from the ogive of the bullet as opposed to measuring from the tip where measurements can be inaccurate (especially with leadtipped bullets)

checkout a company called Stoney Point , they have the items you have heard mentioned here lately (the comparitor, the OAL guage etc) I have recently switched to all plastic tipped bullets, so I find myself using the comparitor less and less. The Nolser line of bullets seem to be right on the money 99% of the time.