Bibasilar refers to the location, predominantly involving the lung bases. Atelectasis refers to incomplete expansion of the lung. More milder forms, such as subsegmental atelectasis, involve only small portions of the lungs; this is quite common and more of an incidental finding, as in your case. More severe forms, such as lobar atelectasis, refer to collapse of one or more lobes of the lung; this is rare but when present may signal central obstruction due to mucoid impaction or tumor.