With regards to medicine and the human anatomy, lymph nodes are small lumps that lie along lymph vessels in the lymphatic system.
Their function is to house cells from the immune system which become activated when they detect pathogens flowing past them in the lymph. This is why your "glands" (nodes) become swollen when you have an infection.
Lymph nodes are also important in surgery. Cancer usually spreads via the lymphatic system so involvement of the lymph nodes draining a particular spot indicates that the cancer has already spread past that point. The draining lymph nodes are often removed concurrently when doing surgical resections of tumours.
Surgically removing lymph nodes can have the unfortunate effect of causing lymphoedema on the affected side because the flow of lymph has been disrupted. Women who have had a malignant breast lump removed and also had axillary node resection on the same side sometimes have swollen arms because of this.