Can you tell me what is interstitial lung disease?

Many diseases. Interstitial lung disease (ild) is a term used to describe many different diseases that affect the lung tissue, or alveolar spaces (as opposed to the airways, like asthma and bronchitis). These diseases have literally dozens of causes, and nearly as many names. By far the most common is "pulmonary fibrosis", itself a name for what is probably many different diseases. If this is confusing...
A lung condition. Interstitial lung disease is a category of lung disorders where the lungs are filled with various cells, scarring, and inflammation. It can be chronic scarring, or an inflammatory type reaction, or destruction of the lung tissue. There are many causes, and thus, if you think you have it, you should be seen and followed by a lung specialist. Prognosis depends on the type of lung disease.