RowBox[, , ... ] represents a row of boxes or strings in input and output.
RowBox objects are generated automatically to correspond to each operator and its operands in input given as \(input\).
The default arrangement of RowBox objects in \(input\) is based on operator precedence. Additional \( ... \) can be inserted like parentheses to specify different arrangements of RowBox objects.
The boxes or strings in a RowBox are output in a row with their baselines aligned.
In InputForm, RowBox objects are output using \( ... \).
In StandardForm, explicit RowBox 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: SequenceForm, GridBox, AdjustmentBox.