I was diagnosed with enlarged retroperitoneal lymph nodes does that mean cancer is present?

No. To proper answer your question we need to know more than the information you have provided. What is your age and your prior medical history? How were those enlarged lymph nodes diagnosed? To prove presence or absence of a disease in those nodes you will need additional work-up and most likely a biopsy.
Possibly. Enlarged retroperitoneal lymph nodes are, for the most part, abnormal. Depending on their size, location, and the remainder of your physical exam, you may need a biopsy procedure to further evaluate. This is really the best way to figure out why they are present, and if they require additional therapy.
Monitoring Centers. Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.