RadicalBox[x, n] represents in input and output.
Inside \( ... \) RadicalBox[x, n] can be input as \@x\%n.
In a notebook a RadicalBox can be created using @ or 2, then using % to move to the index position. moves out of the radical.
In StandardForm and InputForm, RadicalBox[x, n] is interpreted on input as x^(1/n).
The baseline of RadicalBox[x, n] is taken to be the baseline of x.
If RadicalBox[x, n] does not fit on a single line, it is output as x ^ (1/n).
In StandardForm, explicit RadicalBox 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: SqrtBox, OverscriptBox, GridBox.