The Peak Flow meter, or PEF, is a meter you blow into to check your lung function. It measure your ability to breath.
According to National Jewish Health, every asthmatic should blow into his pf meter twice a day first thing in the morning before you take any meds, and before bed. After two weeks, you take the highest number that you blew and this is your personal best.
Now, when you blow 80-100% of your personal best, you are good to go. When you blow 60-80% of your personal best, you should use your rescue inhaler, wait 20-30 minutes, and blow in your pf again. If your pf is now above 80%, you are okay for now, but you should use your pf every four hours.
However, if your PEF is still below 80%, you should call your doctor.
When you blow in your meter and your pf is less than 60%, you should use your rescue inhaler and then have someone take you to the ER. Or, if you are bad enough, call an ambulance (you should avoid driving yourself to the ER).