The remailer performs certain automated tasks which include retrieving mail, decrypting/processing that mail (only mail that is properly encrypted and formatted), obeying the directives within the message and, finally, delivering - remailing - the finished product to a second party in anonymized form. When received by that second party it will reveal only that it was sent from an anonymous source (usually the remailer's name and email address). The IP address shown will be the IP address of the remailer machine.
Using a chain of remailers you can send messages totally anonymous, but you can receive too with a nym, download web pages, send files in FTP, talk in newsgroups, etc...
Remailers protect the privacy and the free speech on-line, because many surveillance systems exist, from marketing to military purpose. The European Parliament scientific unit (STOA) has written up an appraisal of the technologies of political control, beyond the creation of the temporary committee on the ECHELON interception system.
How? Use this tool: https://riot.eu.org/anon/remailer.html.en