The hilum is the root of the lung towards the center of your chest. Without having access to your images or full report, it is difficult to determine the exact location. A perihilar mass would probably be a lung mass, whereas a hilar mass would probably be a lymph node (e.g., metastasis). Since perihilar and hilar are so close to one another, it can sometimes be difficult to differentiate by imaging. Regardless, your findings are worrisome and warrant biopsy/tissue sampling.
On a separate more academic point, if this were in the lung, a pulmonary nodule is defined as a lesion measuring less than 3 cm whereas a pulmonary mass is defined as a lesion measuring greater than 3 cm. In practice, any pulmonary nodule greater than 8 mm warrants biopsy and/or PET/CT according to guidelines published by the Fleischner Society.