Fpdf( is the F-distribution probability density function.
Since the F-distribution is continuous, the value of Fpdf( doesn't represent an actual probability - in fact, one of the only uses for this command is to draw a graph of the distribution. You could also use it for various calculus purposes, such as finding inflection points.
The command takes 3 arguments: x is the point at which to evaluate the function (when graphing, use X for this argument), numerator df and denominator df are the numerator degrees of freedom and denominator degrees of freedom respectively (these specify a single Fpdf( curve out of an infinite family).
The F-distribution is used mainly in significance tests of variance.
The value of the Fpdf( is given by
Here, B is the [wikipedia:beta_function Beta function].