There are two modes of connection to the FTP server: active and passive.
Passive mode is intended for connection via firewall. If you work with the Internet through a home local network or a local network of the enterprise, surely your administrators use a firewall to protect the network. When working on FTP via firewall, you may get an error like “425 Can't build data connection: Connection refused” or similar. This means that you need to change the settings of the FTP program so that it forcibly uses passive FTP mode to connect to our server.