This can be used to stack something above something else; used in math mode.
\stackrel{top}{bot}
The first argument, top, is typset immediately above the
second argument, bot; the former is set in the same
math style as a superscript.
For example
H$_2$CO$_3$ $\stackrel{\rm heat}
{\longrightarrow}$ H$_2$O + CO$_2$
will produce something like
heat
H CO ------> H O + CO
2 3 2 2
Note that the the \rm declaration is used
so that "heat" is set in Roman face and not math italics.