Android’s Messages app has had an update and it now supports “chat” features. The newly-supported features include read receipts and typing indicators. There is also better support for photo and group messages.
This move by Google is a way of trying to let its users throw out the good old SMS.
Note that the new feature will only work if the recipient of the text also has the upgraded Messages app or an RCS-supported messaging app. To know whether your messages are RCS (Rich Communication Services) or mundane SMS, just look at the compose window, RCS threads will be tagged “chat message”.