What does it mean to have a swollen lymph node?

Inflammation.: Lymph nodes are filters for fluid and debris coming from nearby body parts that is collected via lymphatic ducts. They are filled with immune system cells, lymphocytes and react to various processes from minor infections, irritations like rashes, up to serious malignancies. When the node enlarges, the immune system is dealing with one of these processes. Most of the time, it's something minor.

Enlargement of LN: It means that your body is reacting to something/ process that is going on within your body. Swollen lymph node/enlargement of lymph node can be caused by many different things- like infection ( viral, bacteria, fungal, paracytes etc), inflammation, trauma/iritation, autoimmune disease, malignancy etc.

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.