What does a low density lesion in the posterior frontal lobe mean?

possibilities: A number of things could be responsible--stroke, tumor, infection, etc.. You need an MRI with contrast to look more closely to narrow down possibilities.

Unclear: A computed tomography or ct scan produces images that shows the density of the tissue as gray values - bone is very dense = white, air is not dense = black, fluid and brain tissue are gray. A low density lesion now refers to a region of lower density (= darker gray) that the surrounding (brain)tissue and can mean a lot of things. Depending on the history more workup could be indicated.