Well....  it measures the absolute pressure in the manifold.  :)

For the non-smart-a.s answer - it's used by the computer to determine how
much fuel to feed.  By measuring the density of the air in the manifold, the
computer knows how much air is entering the combustion chambers, and knows
how much fuel to mix in to maintain a proper air:fuel ratio.

Problems with the MAP sensor usually will set a diagnostic code, and would
be fairly far down on the list of possible things gone wrong when an engine
won't hold idle.  The IAC motor that you're looking at is a far more likely
suspect.