FrameBox[box] displays with a frame drawn around box.
In StandardForm and InputForm, FrameBox is by default ignored, so that FrameBox[box] is interpreted just as box would be.
In StandardForm, explicit FrameBox objects are output literally. You can use DisplayForm to see the display form of such objects.
See The Mathematica Book: Section 2.8.10.
See also: StyleBox, CellFrame, ColumnLines, RowLines.