Ever needed detailed information about your Wi-Fi connection? Which channel, band, or connection rate? Well as it turns out, even though there’s an app for that, it’s also build right in to OS X. All you need to do, is hold down the ALT key, while pressing the Wi-Fi icon, and BOOM! there’s you info.
It also shows the mac-adresse of the AP you are connected to. This can some in handy, if your are on a large network with many AP, broadcasting the same SSID.
RSSI is the value for signal strengh, and MCS index is a very advanced way of showing you, the current connection type to the Access Point. More info on this here.