“Lymph” means “clear water” in Latin, and it describes the fluid that flows through the lymphatic vessels and lymph nodes which make up the lymphatic system. The three major roles of the lymphatic system - the reason we need it in the first place - are that it returns fluid from the tissues back to the heart, it helps large molecules like hormones and lipids enter the blood, and it helps with immune surveillance to keep infections from running amok.
So, let’s take a closer look at lymph and where it comes from. The blood in the arteries is under a lot of pressure because it needs to reach every little nook and cranny of the body. The arteries branch out into narrower and narrower arteries, and then arterioles, and finally gets to the capillaries - which have walls that are only one cell thick and are slightly porous. Red blood cells are too big to fit through capillary pores, but small proteins like albumin and fluid can make it through. Every day 20 liters of fluid water and protein - seep out of the capillaries and becomes part of the interstitial fluid between cells. About 17 liters gets quickly reabsorbed right back into the capillaries,
but that leaves 3 liters of fluid behind in the tissues each day. This 3 liters of fluid needs to find a way back into the blood so that the body’s interstitial fluid volume and blood volume both stay constant over time. That’s where the lymphatic vessels, or lymphatics, come in: they collect excess interstitial fluid and return it to the blood. Once the interstitial fluid is in the lymphatic vessels, it’s called lymph.
Now - you may be wondering how there can be 20 liters of fluid seeping out each day if the blood volume is only 5 liters, but remember that the 5 liters is constantly in motion and that it gets recycled over and over in a single day.