You've got a high efficiency furnace so the most of the heat has been
extracted from the exhaust. So it won't rise up the chimney--you have
to push it up.

The blower that does that is called an "inducer blower." When the
furnace gets a call for heat, it starts up the inducer to start
blowing air (either sucks it into the burners ahead of the flame or
blows it out after the flame--different mfgrs do it different ways).
Since the whole point is to create a "draft" up the chimeny (just like
using newspaper in your fireplace to combat the cold air rushing down
the chimeney, the furnace needs to know when it's accomplished that.
The pressure switch tells the controller board that there's sufficient
air movement to go ahead and fire up the ignitor and then open the gas
valve.

If your furnace has a few years under its belt, chances are the
inducer blower is weak. Technicans check that out by running an
amperage draw test on the motor. If it's using too much juice, they
replace 'em.