It means you are working on a Responsive project and the text object you are seeing doesn't have enough room to display all the text. So clicking that small icon at runtime will show you the entire text.
Rod explained why that expand icon appears for any text container (caption and shape). Some tips to reduce its appearance:
Thank you both for your replies. The interesting thing about it is the retangle box is very big and the text is only two lines. I will make the recommended changes.
Thank you again,
Check for trailing spaces and line breaks in that case....