Do you mean this config option?

[server] 
hosts = 0.0.0.0:5232, [::]:5232

That is binding the service to a network interface and port. For example your computer probably has a loopback interface and an Ethernet interface and WiFi interface. And you can bind to an IPv4 and or IPv6 address on those interfaces. Which ones do you want radicale to listen to traffic from and on what port? The example above listens on all interfaced both IPv4 and IPv6 and uses port 5323 on all. Of course that port must not be in use on any interface. Generally using this notation is insecure, but fine for testing. Put the real IP addresses when you’re ready.