Well defined opacities in lung on X-ray can be due to a benign or malignant nodule, tuberculosis,
pneumonia or any infarct or granulomatosis. The opacities can also be diffuse—as seen in as in fluid collection, fibrosis, asbestosis, silicosis, tuberculosis etc.
You will need more specialized tests like CT chest for a confirmed diagnosis. If you do not have any symptoms, there may not be a cause of concern, as this may be a benign nodule that may just need an annual follow-up.