This means that any app that uses Google Hangouts won't be available after that date. Google plans on using Hangouts as a tool for meetings which means that Google will be aiming it at enterprise use.
There are some apps that will still work with Hangouts past the April 25th drop dead date. Those include the the DialPad and RingCentral apps that can be used to dial into a call. Hangouts will also continue to work with enterprise communication tools like Slack, and with Google's own Hangouts on Air broadcasting tools like Toolbox, Control Room and Cameraman.
source: Google via TechCrunch