Asthma is one of a spectrum of respiratory illnesses known as Chronic Obstructive Airway Disease (COAD). It is the result of what we call a hyper acute immune reaction.
The lining in our respiratory system is covered in cells whose role is to keep the airway clear of blockage. Thus if the cells detect a foreign body they start an immune reaction that results in coughing and production of either tears, saliva or mucus to trap the irritant. Small hairs then waft the irritant up the respiratory system towards the nose and mouth. Additionally the airway temporally narrows to prevent the irritant form dropping further down. You then either spit it out or swallow it and the airway becomes clear.
Sometimes however the system is too active and gets activated by very small irritants that usually do not affect others eg smoke particles or pollen. Such a person then develops the above state of events to any small change in the environment. Such a person then gets frequent narrowing of the airway and feels unable to breath.
The problem with asthma is that if left unmanaged, it can limit the quality of life you live. It can limit your participation in sports or swimming. However with modern medications people with asthma are able to control the immune reaction and hey also learn what irritants to avoid.