RCS, short for Rich Communication Services, has been aiming to replace SMS for years. But unlike the universal SMS standard, there are multiple RCS implementations that don't work with each other. Google has been trying to solve this, by adding RCS support to Google Messenger and working with carriers to implement Jibe's 'Universal RCS Profile.'
Right now, only Sprint and Rogers support the Jibe RCS profile, but T-Mobile may be next. Multiple T-Mobile customers on Reddit, Twitter, and Google+ have reported seeing the RCS activation popup in Google Messenger.
T-Mobile is no stranger to RCS, with the company announcing its own implementation called 'T-Mobile Advanced Messaging' back in 2015. It remains to be seen if Advanced Messaging will be abandoned in favor of Jibe RCS, but hopefully an official announcement isn't too far off.