Simply put, it means that the animal spent its life grazing and living in open, green pasture. Commercial meat is raised using a confinement model. This packs as many animals as possible into a large space. Animals in these environments are highly stressed and require large amounts of antibiotics to suppress disease and promote fast growth. They are also fed a non-natural diet that is geared towards quick weight gain versus a healthy and nutritious end product.
Although pasture raised meat doesn’t mean the same as organic, generally speaking, pasture raised animals are raised in an organic type of environment. They don’t need antibiotics to keep them healthy because they live in a low stress, natural setting. Grazing on pasture in and of itself is about as natural and organic as it comes.