A hypoechoic liver mass is an area that looks darker than the surrounding liver on an ultrasound scan because it reflects fewer sound waves. Hypoechoic describes how the tissue appears on imaging, not what it is.
Such areas can be harmless (for example focal fatty sparing or some benign growths) or can be tumors, including metastases or hepatocellular carcinoma, so doctors usually confirm the cause with CT, MRI, blood tests, or a biopsy.
Source: Echogenicity, Wikipedia