Bar none basically means without exception.
The phrase is used to emphasize that a statement is completely true and often is used at the end of a phrase or sentence -- nearly always set off with a comma or commas.
Here are some examples in context:
She is the brightest student I've ever known, bar none. [=I have never known a student who is brighter than she is]
We have the best stadium, bar none, in college football.
This is, bar none, the best ice cream I've ever had.
My brother-in-law is the single most boring man on the planet, bar none.