http://en.wikipedia.org/wiki/Checksum

A checksum is basically a mathmatical representation of the file prior to copying.  It is a known value and checks it after copying to ensure it is valid.

Basically the same as a MD5 hash for a file on your computer (virus programs use these all the time).

For example - run.exe (made up file - is hashed and it has a known static value).  If one part of the code is modified or even a space added, the hash will have changed and wont match