The program identifies all of the devices that are using a given WiFi network so that you can see which have been authorized (your own computers, smartphones, tablets, etc.) and which you don't recognize, and don't belong on your network.
The program is quite effective in detecting any unwanted visitors, since it will display both the device's IP address as well as its MAC address and any name assigned to the device.
It doesn't matter if your WiFi is password-protected: these days, there are programs designed to decipher those kinds of passwords and allow other people to connect. Thanks to Who Is On My WiFi? you can stop worrying about the security of your network, since the app will play a voice message each time a new, unknown device connects to your router.
Using it is very simple, you just have to set it to scan your network and then mark all of your own computers and mobile devices as 'Known'. Any other connections can be marked as 'Unknown', and will subsequently not be allowed to connect.