This option allows you to scan for the carrier networks that are available in your area, and connect to one.
Modern phones tend to find the correct carrier to connect to and make the connection automatically, making this section of your settings somewhat redundant, and often a bit clunky. You probably don't need to ever go in here unless you are somewhat curious. 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.