See the following URL for an explanation:
http://computing-dictionary.thefreedictionary.com/Hash+sum

An algorithm that turns a variable-sized amount
of text into a fixed-sized output (hash value).
Hash functions are used in creating digital
signatures, hash tables and short condensations
of text for analysis purposes (see hash buster).
Hash functions are also known as "cryptographic
hash functions." See hash table and one-way hash
function.

If you MD5 or SHA1 hash sum does not match the
one published on the web page, then your
downloaded file is not a valid, usable, .ISO
image; there are errors in the file.