CRC stands for Cyclic Redundancy Check - it's a method of creating a unique reference number for a file so you can verify that you have complete and accurate copy of it downloaded.

You can find more information using google, but here's a starting point....

http://www2.rad.com/networks/1994/err_con/crc.htm
http://www.users.cloud9.net/~stark/ctappc.pdf