How serious are lymph node enlargements?

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.
Depends on cause. Lymph nodes help to fight off a variety of illnesses, including infections, non-infectious inflammations, cancer, etc. The severity depends on what caused them to enlarge.