With an old-style proxy list you held hundreds of separate addresses and switched between them yourself. A backconnect proxy turns that around: you always connect to the same host and port, and the gateway chooses the exit address for you.
This is how rotating residential and mobile proxies work today. The gateway reads instructions from your username, such as the country or a session label, and picks an address to match. On PiratProxies the residential gateway is resi.piratproxies.com on port 8080, and everything else is set in the username.
Read next: How the username controls the gateway