Organic means that your food is "clean." More specifically, crops were grown without conventional pesticides and are GMO free, and animals were reared without the use of hormones and non-therapeutic antibiotics. When you buy organic food, you are supporting a gentler way to raise food on the planet and ensuring that fewer pesticides and other harmful chemicals (like bovine growth hormone in conventional milk) enter your body.
Buying locally grown food is even more important than buying organic food, because locally grown food has fewer miles to travel (saving on environmentally harmful transportation emissions), is grown in season (thus needing fewer chemicals), arrives thousands of miles fresher, and is the key to building a sustainable food economy.
One easy way to reduce your ecological footprint is to increase your intake of vegetarian meals. Growing the crops to feed farm animals requires massive amounts of water and land; more than 70 percent of the grains and cereals we grow are fed to farm animals. Methane emitted by livestock and their manure, equals the global-heating impact of 33 million cars, according to the book Six Arguments for a Greener Diet. A person doesn't need to become a vegetarian to reduce the environmental impact of food choices. Start with where you are, and reduce your meat intake from there.