An open proxy is a server that acts as a "middle-man" for requests from one server or network to another server or network and is accessible by any Internet user without restriction.
Open proxies do not filter, encrypt, or otherwise check what content is coming into servers. Because of this, they can allow malware to be downloaded and infect computers and can be used for the transmission of illegal content and spam.
Avoiding open proxies
SpamCop provides some good resources for fixing open proxies.
