At Par

Most bonds sell in units of $1,000. For all kinds of weird historical reasons, the thousand figure there isn't the primary quote when brokers quote bond prices...rather, everything is out of 100. So, a bond trading "at 99 point 2" is actually likely a thousand dollar bond selling for 992 dollars.

That 99.2 is "below par" and if a bond were "above par", it'd be trading at some value over the quoted 100 or some figure above $1,000.

Why do bonds move above and below par? Lots of reasons. The biggest is perceived risk that the bond won't actually pay off its borrowed principal. That's individual bond risk.

But the overall market can change as well. So, the government perceives inflation to be a strong force on the horizon so the Fed quickly raises prevailing interest rates. And if they do, bond prices fall across the board. So, a bond with a 6% coupon that used to be trading above par (which is to say, above a grand), in this scenario, could drop below par if prevailing rates suddenly go up.