That's an indication that the text is too wide for the frame for some reason. Common causes are the application of No Break to a large block of text (many new users mistake No Break for turning off hyphenation), long strings of text where the words are separated by non-breaking spaces and hyphenation is turned off, Keep options that require all of the text to remain together in a frame that is too small to hold it, or indents, either at the frame level or the paragraph level, that make the column too narrow to hold text.
You can open the story in Story Editor view (Edit > Edit in Story Editor or Cmd/Ctrl + Y) so that you can see the text. The easiest way to figure ot the problem is then to select the first line of overset text -- there will be a red line indicator in story editor -- and look at all of those formatting possibilities.
