File Transfer Protocol, or FTP, is one of the oldest internet protocols still in active use, a standard way to move files between two connected computers, most commonly between a local machine and a web server. An FTP client (FileZilla and Cyberduck are common free options) connects to a server using a hostname, username, and password, then lets a user browse, upload, download, and manage files much like a local file browser.
For website management specifically, FTP is how files get onto a server when there's no CMS dashboard handling it directly: theme files, plugins, configuration files, or a full site migration. Plain FTP transmits credentials and file contents unencrypted, though, which is a real security risk on an untrusted network, so SFTP (SSH File Transfer Protocol) or FTPS (FTP over SSL/TLS) are the versions worth using instead wherever a host supports them, since they wrap the same functionality in actual encryption.
