This option allows you to scan for the cellular network operators with services available in your area, and connect to one.
This setting is largely obsolete now, with modern phones finding the correct carrier to connect to and making the connection automatically, with SIM cards only allowing connecting to a single operator network.
You probably don't need to ever go in here unless you are curious to see the network operators in your area. Once in, you can begin the (sometimes lengthy) process of scanning the radio bands for all the carrier networks available in your area. From there, you will be able to see which ones are available, and which ones (according to your current SIM card) you are allowed to use.
You can then try to connect to one of the networks in the list, or just connect automatically to whichever network your device (according to its SIM card) thinks you should connect to.