On a liver ultrasound what does mildly echogenic parenchyma mean?

Depends: It difficult to say without actually seeing the image and knowing your history. It may simply be a variation of normal given the mild gradation. Increased liver echogenicity can be seen in a context of hepatic steatosis (many etiologies - obesity, diabetes, hepatitis, alcohol, medications side effect including steroids, and even pregnancy). Talk to your doc and correlate with liver function tests.

Fatty liver disease: Increased liver echogenicity is typically due to fatty deposition of the liver. This is most commonly from alcohol intake or obesity. It is important to try to eliminate the underlying cause for the fatty liver. Long standing fatty liver can lead to end stage liver disease.