If you have assigned manual text flow to a text frame, it remains assigned to this text frame even if you clear the text frame subsequently and you can not see anything of the assignment, so to speak.
This icon serves for deleting the flow path and re-establishing rectangular formatting. To do this, just select a text frame where the text flows around an object and then click on this icon. All formatting around any objects will disappear from the frame, and the text will fill it normally so you can start again.
