Asthma is a chronic inflammatory condition of the airways that causes episodic exacerbations of bronchoconstriction. Bronchoconstriction is where the smooth muscles of the airways (the bronchi) contract causing a reduction in the diameter of the airways. Narrowing of the airways causes an obstruction to airflow going in and out of the lungs.
In asthma there is reversible airway obstruction that typically responds to bronchodilators such as salbutamol. This bronchoconstriction is caused by hypersensitivity of the airways and can be triggered by environmental factors.