This article is from the Asthma FAQ, by Patricia Wrean and Marie Goldenberg email@example.com with numerous contributions by others.
A peak flow meter is a little plastic device which you blow hard
into, after having taken a deep breath. It records the rate at
which you've blown into it in litres exhaled per minute (L/min)
-- this is called the peak expiratory flow rate (PEF or PEFR).
The meter is essentially a cylinder with a mouthpiece at one end,
a place for the air to escape at the other end, and a calibrated
meter along the side. When you blow into it, a marker is pushed
along the scale and comes to rest at a point which indicates your
PEF. Since you want to measure your maximum peak flow, it is
important to take a deep breath and blow as hard and as fast as
you can. Many asthmatics find that their maximum peak flow provides
a good objective measure of how their asthma is doing, so peak flow
meters now are used extensively for self-monitoring of asthma, and
also for monitoring the effectiveness of asthma medications.