What actually is asthma ? Is it allergy or or a convulsion or just a moon reflection.?

Asthma is an inflammation AND constriction of the airways of the lungs. This just means the airways get "puffy" which narrows the tubes and makes it more difficult to move air in and out of the lungs. This is why steroids are given for asthma. The constriction (convulsion isn't a term I would use, but whatever) causes a tightening of the tubes/airways in the lungs. This is why bronchodilators are used to treat asthma.
Confusion of allergies and asthma is common because a lot of asthma is triggered by allergens. This just means something that a person is allergic to (like pollen or peanuts)causes the inflammation and constriction and therefore the symptoms of asthma. However, other things like a viral illness or exercise may also be triggers for asthma.
Definitely not a moon reflection:-)