Cage-free is meant to stand for animals, usually chickens and other poultry, that are not raised in cages.
Traditionally in the U.S., a majority of conventional egg-laying hens have been raised in battery cages vs. pastures. The Humane Society of the United States notes that on average, cage confined birds are afforded only 67 square inches of cage space (less space than a single sheet of letter-sized paper) for their entire life. More than simply unkind, cage life doesn't allow for many natural bird behaviors such as nesting, perching and dustbathing, all of which are important and healthy habits for birds.
The Humane Society of the United States supports cage-free birds, and states, "While cage-free does not necessarily mean cruelty-free, cage-free hens generally have significantly better lives than those confined in battery cages."
That said, just like the term Free-Range, Cage-Free is not a term or label that's certified or verified by the government or another independent source. Although cage-free standards are supposedly getting better, right now, seeing cage-free as a label, say on a carton of eggs, isn't enough to tell you how the bird were really raised. The term doesn't officially explain if the birds were allowed outdoor access or if they were let out of cages sometimes, but not all the time.
The best way to know how birds are raised is visit local farms, see for yourself how the chickens are raised, and if the conditions are good, you can buy eggs from that specific producer.
How organic certification is connected to cage-free
NOP has a set required origin rule that says that organic poultry must be under continuous organic management starting no later than the second day of life. As organic livestock, organic poultry, with few exceptions, are required to have some access to pasture. NOP states in §205.239 livestock living conditions that organic producers must allow, "Year-round access for all animals to the outdoors, shade, shelter, exercise areas, fresh air, clean water for drinking, and direct sunlight, suitable to the species, its stage of life, the climate and the environment."
Currently, the access to pasture rule is much more clearly defined for ruminants vs. birds, but still, beyond checking out local farms for yourself, the USDA Organic Seal is another way to help ensure the eggs you're purchasing came from cage-free birds.