You can download mand5 from the market, and use it to check the md5 of the file. The number should be the same as the one that is listed for the file (typically from wherever you downloaded it from).
It basically checks the integrity of the file, so you can make sure it's not corrupt.
If flashing through fastboot, it kinda redundant, because fastboot will check the file also...
Sent from my Nexus One using XDA App