This is a small hand-held tube that can measure the flow of air coming from your breathing tubes.  You can have one at home and measure your peak flow by blowing into it as hard and fast as possible.  You will be shown how to use it. It can be useful in managing asthma. If you perform a test when you are very well the recording  serves as a useful reference to compare future recordings with if you are developing worsening asthma symptoms. In this way your doctor may help you develop a management plan that helps guide you as what to do if you start to develop an asthma attack.