What is the app that lets you see Wi-Fi signal through your house?

Excuse me, could you possibly elaborate on the application that supposedly allows users to visualize Wi-Fi signals throughout their residences? Is this a legitimate technology, or perhaps a misconception? If it's indeed a real app, how does it work? Does it require any special hardware or permissions? Are there any privacy concerns associated with using such an app? Additionally, could you recommend any specific apps that might fit this description, or perhaps suggest alternative methods for identifying and optimizing Wi-Fi coverage in one's home? Thank you in advance for your insights.