The term "lower lobe" refers to one of the sections (lobes) in the lower portion of one of the lungs. The term "infiltrate" refers to the presence of pus. So whoever has this lower lobe infiltrate has pneumonia in the lower part of one of his or her lungs.
Are you referring to a lower lobe lung infiltrate? It is a term used to describe the appearance of a lesion on an x ray. A spot on the lung.