It's saying that that text frame has “#” characters too many to fit into the frame as designed. If the user clicks into that frame they should delete “#” characters. The designer must have fixed the size of the text frame. So, for example, in the interactive editor what would happen is essentially the same thing except the notification would be a red square with a white plus sign in the lower right corner of the text frame to let the user know that they had added too many characters.
If the designer had defined the text frame as unlocked or used the "Fill Text Frame" option then the user could add more text.