Answer:
A white spot on an x-ray of the lung is also called a pulmonary nodule. A spot less than 3cm is a nodule, and larger is a mass. There are a couple reasons a pulmonary nodule would show up in the lung field.
First, an infection in the lungs has left scarring, or a benign nodule in the lungs which does not need treated. Typically these types of spots will show up on the x-ray with defined borders. The other type of nodule is a malignant, or cancerous, pulmonary nodule. These need to be immediately treated for the best possible outcome. They typically show up on an x-ray with fuzzy or hard to define borders.
It is recommended that if you smoke or are at risk for cancer to have annual chest x-rays done for precaution to ensure that any malignant nodule is caught before growing larger, becoming metastatic, and spreading.