The term hypoechoic is used when describing how an object looks and behaves while being examined by the ultrasound
technique. Hypoechoic literally means that it does not bounce back sound waves very well (does not echo the sound). This is a very non-specific finding that means that you have a mass which needs to be further looked at by either more detailed imaging (MRI or CT scan), or it needs a biopsy so the tissue can be looked at in more detail.
Hypoechoic masses in the liver can be one of several things. Liver abscess
which are collections of an infection can cause this. Certain liver tumors including the hepatic adenoma and tumors that have metastasized to the liver can cause this type of mass. While these are possibilities, there are many types of liver masses including the hepatic angiomas which are benign and are left alone. Since there are many different possibilities with different outcomes, it is important that you follow this up with your doctor
I suggest that you schedule an appointment with the physician that ordered the ultrasound. He or she will be able to discuss the different possibilities this mass could be in the context of you past medical history and current physical exam. From there, you can discuss the plan for further evaluation (imaging or biopsy). Good luck.