It allows anyone on the internet (or gatewayed network) to access a SUBSET of your files. On a unix system, this involves setting up a user "ftp" in whse home directory the public files will be. An external user sees a file system whose root is that home directory. Look at the man page for ftpd on your system.
Normally, the public files are kept in /pub or a subdirectory of that.
Any file with which is accesible using anonymous ftp is accesible as a W3 document with address