It is the option to use an FTP site without having username or a password. You can use Anonymous FTP on your web hosting account to allow users to upload and download files from your site. However you need to use this feature carefully as a malicious user can upload MP3 files or pirated software and your site may be shut down for violation of the web hosting company terms. So in our opinion nobody should use Anonymous FTP - it is just not safe.
Downloading files from a public-access Internet machine, i.e., one which allows a remote user to log in as "anonymous" and transfer files using the ftp protocol even if the user does not have an account on the machine.
Using the FTP function of the Internet without a secret login ID and password. This is permitted on large systems that share some of their files with outside users who otherwise would not be able to login.