As you know, when you are on a Teams meeting you can send message using the chat option.
Well, until then participants had to manually open the chat window when somebody send a chat.
This is no longer the case (the deployment is underway and may not be yet available on your tenant); a so called ‘chat bubble’ will pop up on top of the main screen to all participant, showing the message sent.
NOTE you have to be quick as it disappear after few second.
You can then click on this bubble to open the chat window if needed.
If you want to turn off this bubble on your side, open the ellipsis menu (you know the 3 dot) and click on Don’t show chat bubbles