N icon on your Android display appears when utilizing the integrated NFC feature, and a rectangle with a Wi-Fi icon shows up during screen mirroring sessions. Another icon in the status bar you might see is one that resembles a key.
The key icon on your Android display signifies that you are connected to a VPN service. VPN — which stands for Virtual Private Network — provides a secure connection that protects you from online tracking and permits access to websites restricted by region. This is facilitated by concealing your actual IP address and utilizing the IP address of the VPN server. Every occasion you activate the VPN service on your Android device, a key icon will be displayed in your status bar. However, you can easily turn it off as well.