Google Hangouts is a communication platform accessible via web site, browser plug-in, or mobile app. Questions are likely to be off-topic here unless they involve hardware or networking issues, or a software problem such as an installation or configuration issue. They may be on-topic at WebApps or the appropriate site for the mobile application.
Google Hangouts is a communication platform developed by Google which includes instant messaging, video chat, SMS and VOIP features. It replaces three messaging products that Google had implemented concurrently within its services, including Google Talk, Google+ Messenger, and Hangouts, a video chat system present within Google+.
Hangouts allows conversations between two or more users. The service can be accessed online through the Gmail or Google+ websites or via a browser plug-in, or through mobile apps available for Android and iOS.
Source: https://en.wikipedia.org/wiki/Google_Hangouts
Questions may be on topic here if they involve software issues, such as problems installing or configuring a browser plug-in, or hardware or networking issues, but questions about the service, itself, or using it are likely off-topic.
Questions regarding the web-based version of Hangouts should be redirected to the Web Applications Stack Exchange when relevant.
Questions regarding the mobile application may be on-topic at the appropriate platform site, such as Android Enthusiasts Stack Exchange, or Ask Different Stack Exchange for iOS.