what is a radar note?

This sounds really ignorant of me but what is a radar note? Is it something I know about but just don't know that it's called that? Please somebody explain!

This is part of a reply to a similar question from another user a few months back (answer compliments of Idjit):

Radar bills: are bills where the serial number reads the same both ways- like the word 'radar', e.g. ABC1234321, or ABC2321232. People like to collect these, so DR (the sites programmer and sponsor) programmed a warning into the bill entry screen that tells you the bill is a radar in case you missed it. If it's in good shape, you can sometimes sell it to someone on the site.

Thanks for explaining!