Chest ex ray says lungs show increased interstitial markings with no active infiltrate,mass or effusion. what does that mean ?

Depends on context: The interstitium of the lung refers to the area in and around the blood vessels and alveoli (air sacs). This is where the exchange of oxygen and carbon dioxide takes place. Inflammation and scarring of the interstitium disrupts this tissues. So the radiologist is noting that there appears to be increased markings in the area. There are many different conditions and disease that cause this pattern.