What Does Sub Segmental Bi Basal Atelectasis Mean?

I just received my chest x-Rays report today and could you please explain sub segmental bi basal ateclectasis? My lungs are low in volume and don t understand. The cardiac silhouette is within normal limits in size and I have no visible pleural effusion or pneumothorax. I have no systems regardinding my lungs but my two feet and ankles swelled up last week and did not change with a prescribed water pill. Thank you