Lymph nodes are merely sentinels for the lymph which flows through our body via the lymphatic system.
Lymph itself is all the blood plasma that has seeped out of our blood vessels into our tissues, then drained away via lymphatic vessels. Lymph also contains fats that have been digested through our gut after a nice fat laden meal.
Lymph nodes sample the lymph-sludge that flows past, checking to make sure bad things are not also flowing through like cancer cells, bugs and the like.
The cells that do this sampling are called lymphocytes.
(-cyte meaning "cell").