From The Lexicon of Sustainability:

Livestock that forage freely on grass and legume pastures as oppose to being fed corn and grains in confined feedlots.

Instead of feeding their animals a diet of grain, farmers like Don Gilardi of Red Hill Farm are turning their animals back onto the land and offering them a diet of grass and alfalfa. The results are richer flavored products, whether it be for cheese, milk, or meat. 

(Learn more about grass fed at the Lexicon of Sustainability)