What he means is not centered all the way round. I've had one or two like that and while I don't recall it having any adverse effect it used to drive me up the wall as it made it look like the wheel was miles out of true.
They way @Phaeton describes the problem is that one side of the tyre is further forward/back then the other, I think.

 
				 
 
		 
 
		 
 
		
 
 
		 . it felt like the tyre was trying to roll off the rim.  Different construction, different characteristics.
. it felt like the tyre was trying to roll off the rim.  Different construction, different characteristics. 
 
		 
 
		 
 
		 
 
		