Part of the standard Basic Calculations palette.
An International Characters palette for European characters.
Clicking the button inserts an into your notebook.
Here is a working version of the palette.
In a palette like this, the indicates where the current selection should be inserted.
Clicking the button takes the highlighted selection and wraps a square root around it.
Palettes can be part of a notebook or stand alone.
Clicking the button immediately factors in place the expression you have selected.
Follow this link to see other examples of palettes.