A mitered border looks like this:
Whereas the "blunt" border is like this:
(not my quilts, btw, though I've made plenty of both styles; I found these on the 'net)
The mitered border is a bit of a skill to achieve well, but not that hard. It's especially called for if the border has stripes or a pattern It's not totally necessary if the border is a solid color and/or so quilted that the seam is hard to see anyway. However, I miter almost all of my borders these days just because I think it creates a finer, more professional finish.
Good luck!
--Q