Scan for Wi-Fi networks
The Scan option requests a re-scan of the nearby available Wi-Fi access points.
Your device will scan nearby access points when opening the Wi-Fi settings screen anyway, as well as on a regular basis while Wi-Fi is on, so this option is rarely necessary. It might be of use if you have just turned on a home Wi-Fi access point or router and you want to immediately re-scan for nearby access points.
Hidden Wi-Fi networks
It is possible for Wi-Fi access points to be active but not to advertise their availability to devices. Devices may connect as normal if they already have been configured, but new devices won't see the Wi-Fi network in their list. When this is the case, you will need to be able to manually establish a connection to the network on your device using the Add network option, by typing in its network name and choosing its connection settings instead of selecting them from a list.