The \raisebox command is used to raise or lower text. The first mandatory argument specifies how high the text is to be raised (or lowered if it is a negative amount). The text itself is processed in LR mode.
Sometimes it's useful to make LaTeX think something has a different size than it really does - or a different size than LaTeX would normally think it has. The \raisebox command lets you tell LaTeX how tall it is.
The first optional argument, extend-above, makes LaTeX think that the text extends above the line by the amount specified. The second optional argument, extend-below, makes LaTeX think that the text extends below the line by the amount specified.