Lymphoma is a type of blood cancer that develops when white blood cells called lymphocytes grow abnormally. Lymphocytes are part of your immune system. They travel around your body in your lymphatic system, and help you fight infections and cancers. There are two types of lymphocyte: T lymphocytes (T cells) and B lymphocytes (B cells). DLBCL affects the B cells. DLBCL involves the abnormal growth of B cells.
There are lots of different types of lymphoma. DLBCL is the most common type of non-Hodgkin lymphoma. It is a fast-growing (high-grade) lymphoma.
It is called diffuse large B-cell lymphoma because:
it develops from abnormal B cells
the abnormal cells are larger than normal, healthy B cells
the abnormal cells are spread out (diffuse) rather than grouped together when they’re examined under a microscope.

I hope this helps you understand better.