It refers to an option wherein files can be downloaded without having to register and create an account.

Few host agencies that provide FTP services offer you with an access to anonymous FTP. If you are a user you can avail these services without having to open account or as an “anonymous” user while prompted to use a user name in the beginning. In most cases, users are required to supply their email address to avail a password. However in this case, no such supply of information is required. The FTP client will have to supply dummy data as a password as it hides the actual login process from the user. An alternative to anonymous FTP is the Gopher protocol.