What is retroperineal lymphadenaphy?

See answer : it is a collection of enlarged lymph nodes located in the area anterior to the spine and posterior to the intraperitineal organs (most small and large bowel, liver, spleen etc). It can be due to a reaction to infection, lymphoma, other malignancy, inflammatory disorders, autoimmune disorders, and many others including idiopathic (meaning no known cause).

Lymphadenopathy: is enlarged lymph nodes, for which there are many potential causes. The retroperitoneum(I assume that's what you mean) is an anatomic space behind the peritoneal cavity(which contains most of the abdominal organs and bowel). The retroperitoneum contains the pancreas, kidneys, and aorta. So retroperitoneal lymphadenopathy means enlarged lymph nodes located in the retroperitoneum.