Third-party apps cannot support RCS at this time.
RCS is the next-generation messaging protocol meant to replace SMS/MMS in the future. It is more reliable, web-based, allows for larger media files, supports things like reading receipts and typing indicators, and will just be all-around better. It is on a limited rollout throughout different carriers, devices, and applications. A general Google search can provide you with more information. No one knows when or if it will ever fully replace SMS/MMS, but that is the goal.
For third-party app support (such as Pulse SMS), Google will need to release a way for developers to integrate it. They have not done this and have made no indication of when they will. We do expect them to, at some point.