File Transfer Protocol - standard network protocol for transferring files between computers. Like having a specialized mail service just for sending packages.
Web developers use FTP to upload website files from their computer to a web server.
FTP is an Internet protocol, not a managed cloud service. All major cloud providers can host FTP servers on virtual machines or containers, or you can use managed file transfer services that support FTP/SFTP/FTPS for moving files into cloud storage.