I must take issue slightly with tyketto ... as I understand transition level, it is identically equal to what FAR 91.121 calls "lowest usable flight level", so is not constant in the US, but varies with local altimeter. 91.121 has a table that specifies transition level varying from FL180 to FL210. The transition altitude is 18000 MSL anywhere in the US.
To the original poster, the reason why a flight level may be unusable is that at low local atmospheric pressure, you would find that if you set your altimeter to standard pressure and fly 18000 on the altimeter, which is how FL180 is defined, your true altitude drops below 18000 and you are in conflict with traffic flying below 18000. The solution is to block out use of any flight level that results in a true altitude below the transition altitude. The saying is "High to low, look out below".
The same applies if you are on an instrument approach. If you are flying it with an altimeter setting that is higher than actual conditions, you are too low and may hit something. This is why some approaches say, "when local altimeter setting not received, use (some other) altimeter setting and increase (specified minimums and visibilities)" to compensate for the fact that you may in fact be too high or too low.