What does it mean if my chest X-ray showed a 12 mm nodule?

Infection/tumor. Nodules can be related to granulomatous infection such as tuberculosis or coccidiodomycosis or brucellosis if live in endemic areas. Other nodules can some times be caused by staph and can cavitate. Nodules can be seen in metastases and primary lung tumors..
Young? Non-smoker? Doctor will decide on follow-up depending on the risk of cancer. Probably less than half of the solitary nodules are cancerous, and the risk is likely even lower in young non-smokers. Doctors may recommend more studies such as repeat chest x-rays, chest ct, or biopsy. Tuberculosis, aspergillosis, coccidiomycosis, cryptococcosis, histoplasmosis, etc... Can cause a lung nodule.