There are linear densities in the left basal lung field. What do mean by this?
By left basal lung field they mean the bottom of the left lung.
In an x-ray, areas that are more dense (from fluids, tissues, etc) show up as light streaks. They see lines in your x-ray.
It can indicate an infection, it could also be from the x-ray itself. More likely however, it indicates atelectasis. This is an indication of part of the lung collapsing. It can indicate a cardiac condition.
See your doctor. Good luck.