There are plenty of technical resources on the Web that discuss cyclic redundancy checks (CRCs). Most times you won't need to worry about this technobabble. That is, until one it day it suddenly appears and you think - what the hell does that mean? In simple terms, a CRC is bit of mathematics used to ensure that your data is OK when being transfered. It's a checking procedure that quickly identifies when data has been damaged. If you get this message, it means that the file being read by your PC or software is corrupted. However, it does not mean all the data is lost forever. When data is transfered, it is usually in small blocks and each block is given a CRC value. If something goes wrong with the data between the time it leaves the source and arrives at its destination, the CRC sent at the source will no longer match the one that is calculated when the data arrives - this is when the cyclic redundancy check error will appear.
The most common times you will see the cyclic redundancy check error message is when trying to read data from a damaged CD or DVD. Just before it appears, your CD/DVD drive will probably grind and whirl away - your PC may also become a little slugglish.