GMO stands for “genetically modified organism”. Sometimes you'll see "GE" in this context, which stands for "genetically engineered".
Farmers are not allowed to save seeds from their GM plants, so they must buy seeds and special pesticides every year.
GM plants cross-pollinate with other plants, giving control of our food to corporations by undermining our ability to grow, sell, and eat organic food. When corporations rule our food, we lose food security, plant diversity, small family farms, and the choice of what we grow and eat.
GM plants are banned in Germany, Ireland, Hungary and many other places. Many US and Canadians regions are now declaring themselves GE or GMO Free. Canada does not label GM foods, although there is a Non GMO Project where food producers can be verified as meeting the Project's Standard.