That symbol means that the rectangles that are drawn above the type areas have been made into buttons using the Buttons and Forms Panel. These buttons will work when the document is exported to an Interactive PDF (File>Export>Adobe PDf (Interactive)) and then opened in Acrobat versions 9.0 or later. They will not work in a Print PDF or with most other PDF readers. Using a blank rectangle as an interactive element can also be done using the Hyperlinks Panel. In that case a Print PDF will read the interactivity and there is more of a chance that PDF viewers other than Acrobat will be able recognize it.