This option in the drop-down menu requests a re-scan of the nearby available Wi-Fi access points.
Your device will scan nearby access points anyway, and 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.
Note that some access points can be configured not to advertise their availability to nearby devices - a setting often called "SSID broadcast" that is made who-ever set up the access point itself. Such access points will not appear in the list of available access points, but it may still be possible to connect to them if you have connected to them previously and they still appear in your device's list of remembered access points, or by manually entering their details using the Add network menu option.