According to New Geography, "think globally, act locally" is a slogan encouraging people to think about the global ramifications of their actions while making an effort to improve things locally. The phrase has been used in a variety of different fields and has many interpretations, but it is commonly associated with the environmental movement.
According to The Telegraph, conservationist David Brower, founder of Earth Day, was the first to coin the phrase. The principle behind that inaugural Earth Day on April 22, 1970, "think globally, act locally," became the mantra of the environmental movement, as noted by the The New Republic.
In ecological terms, "think globally, act locally" recognizes the fact that environmental protection is a global problem, but one that average citizens can address by making efforts in their local communities. For instance, the problem of trash in landfills is an enormous one from a global standpoint, and a single person might feel powerless to address the issue. But by making small changes in one's own life, such as reducing waste and increasing recycling efforts, an individual can do his or her part toward solving the problem. If enough people pool their efforts on a local level, the effects is much greater.
Beyond its environmental conservation implications, the phrase has been adopted in the business world, and more recently by the Millennial Generation. New Geography points out that the phrase exemplifies that generation's outlook towards implementing societal changes on a direct, local level and their belief in changing the world one community at a time.