What is lymph?
Lymph is plasma seeped out of capillaries.
Blood vascular system is blood circulated through arteries capillaries and veins. At the capillary end plasma seeps out of capillaries. Capillaries are endothelial cells. Through endothelial cells some of the plasma easily leaks out of the capillary wall.
In short all the blood coming of arteries is not returned to heart through veins.
The leaked plasma out of capillaries is the lymph. It is plasma without red blood cells.
Lymph is collected though lymphatic system and poured back in circulation.