A Check Sum is a sum of some sort that is generally used to verify if a file isn't corrupted. The simplest checksum simply reads a file and adds all of the bytes of the file together. For example, in Scheme:
(define file "")
(display "Please Enter a filename to read: ")
(set! file (read))
(define sum
(let sum-loop ((current-sum 0))
(let ((current-char (read-char file)))
(if (eof-object? current-char)
current-sum
(sum-loop (+ current-sum (char->integer current-char)))))))
(display "The simple checksum of the file is: ")
(display sum)
(newline)
Of course, that simple checksum algorithm isn't terribly useful because it is possible for more than one file to share the same checksum. There are more complicated checksum Algorithms that guarantee the uniqueness of the checksum. A better checksum function would use specialized HashFunctions to produce a unique sum.