Well, it's not a tool... The MD5 checksum is a verification method that exists for security. Every single file has an MD5 based on it's file size and contents - it's all summed up into 20-some characters.
Some download websites will supply the MD5 checksum of each downloadable file so users can verify that they have received the correct file - and that the site did not redirect them to another file by mistake.
Hope that sums it all up for ya