Iwconfig is similar to ifconfig(8), but is dedicated to the wireless interfaces. It is used to set the parameters of the network interface which are specific to the wireless operation (for example : the frequency). Iwconfig may also be used to display those parameters, and the wireless statistics (extracted from /proc/net/wireless).
All these parameters and statistics are device dependent. Each driver will provide only some of them depending on hardware support, and the range of values may change. Please refer to the man page of each device for details.
This video show iwconfig run on Raspberry Pi 3/Raspbian Jessie with build-in WiFi, to display WiFi info:
We can get WiFi link quality with command:
$ iwconfig wlan0 | grep -i quality