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 open standard protocol, not a cloud provider-managed service. All major clouds can host FTP servers on VMs/containers, but providers more commonly recommend managed file transfer options (SFTP/FTPS via partner products) or object storage transfers instead of plain FTP.