Several things. Lymph nodes are a site of inflammatory response and immune reaction. Swollen, tender lymph nodes are a common response to bacterial or viral infection. Lymph nodes can also enlarge and become tender in certain autoimmune diseases. Lymphoma is a type of tumor that develops in lymph nodes and finally certain types of cancer will "drain" cells into a lymph nodes and spread to those nodes.
Infection. Typically, a tender swollen lymph node means your body is trying to fight a localized infection. Most of the time, these events are self-limited. If you are having fevers, the tenderness increases, and your skin overlying the lymph node becomes extremely red, you will need to see you doctor for additional treatment.