Google has been beefing up the Messages app allowing it to work with many of the features available with Rich Communication Services (RCS). This means that messages are sent through data networks instead of a carrier’s cellular network allowing messages to be sent over Wi-Fi. At the same time, users can fill each message with as many as 8,000 characters instead of the previous limit of 160. And Android device owners will get a read receipt to confirm that their messages have indeed been read.
Not all Android users have received the update. It appears to be disseminated to Android users via a server-side update so just keep your eyes peeled.