Blood vessels? "hypervascular" means "many blood vessels." you must be referring to a ct or MRI scan with contrast, where the radiologist is referring to a high amount of contrast localizing to the spot in the liver. The question people want to know is "what is it? " and sometimes that's not possible to know on a simple scan alone, further tests may be needed.
Cells and vessels. A hypervascular mass in the liver is a tumor, which is made up of cells (benign or malignant) and their support structure (fibrous stroma and blood vessels). The term "hypervascular" is a radiologic term that refers to the way the tumor fills up with contrast agent on contrast enhanced imagining studies (ct or mri). All "hypervascular" masses need to be rigorously investigated to rule out cancer.