It refers to the experience of seeing firsthand the reality of the Earth in space, which is immediately understood to be a tiny, fragile ball of life, "hanging in the void", shielded and nourished by a paper-thin atmosphere. From space, national boundaries vanish, the conflicts that divide people become less important, and the need to create a planetary society with the united will to protect this "pale blue dot" becomes both obvious and imperative.
Third-hand observers of these individuals may also report a noticeable difference in attitude. Astronauts Ron Garan (Ted Talk on YouTube: TedxVienna - Ron Garan - The Orbital Perspective of Our Fragile Oasis) Rusty Schweikart, Edgar Mitchell, Tom Jones, and Mike Massimino are all reported to have experienced the effect.
The term and concept were coined in 1987 by Frank White, who explored the theme in his book The Overview Effect — Space Exploration and Human Evolution (Houghton-Mifflin, 1987), (AIAA, 1998). The overview effect has been considered to be one of the stimuli that led to the Gaia hypothesis.
An article claims the environmental protection movement was inspired by NASA's Earthrise (and another) photograph.