Asthma happens when the tubes that carry air into the lungs (the bronchi and bronchioles) become narrowed. This makes it harder to breathe - like trying to suck air through a straw. Asthma can be caused by infection (bad coughs and colds) or because you're allergic to something (pollen, cat or rabbit fur are common examples).
Asthma can be easily treated with medicine taken by inhalers, and sometimes as a liquid medicine to swallow. These medicines help the tubes to open up and stay open, so breathing is easier. All asthma medicines are either relievers (that help the tubes open up when they're tight) or preventers (which you take regularly to stop the tubes getting tight in the first place). Different people have to take these medicines in different ways to get the best results from them. It's important to follow the instructions and use them regularly to stay free of trouble. Not taking asthma treatments can mean spending a lot of time being unhappy because of breathing problems and coughing a lot.
The great news is that if you get asthma when you are young, it's likely to get better (and may one day go completely) as you get older.