The Green Revolution was a program of agricultural development that was largely responsible for the worldwide increase in agricultural output in the 1960s and 70s. It was mostly due to the ideas and work of Norman Borlaug, an American agronomist who worked on developing more productive strains of common grain crops.
In the 1940s, Mexico was looking to improve the agricultural output of local farmers. The U.S. Department of Agriculture provided a team of American scientists to help with the project, and eventually sent over Borlaug, who started working on the problem of stem rust. Stem rust weakens the stems of wheat plants, causing them to fall over, ruining the wheat berries growing atop the stem. Borlaug bred a type of wheat with a stronger, shorter stem, which also produced more wheat per plant. This variety also turned out to be hardy across multiple different soil conditions and climates, proving to be a boon to farmers around the world. The Rockefeller Foundation supported an equivalent program to produce high-yield rice to help feed Asia. South America and India were largely able to feed themselves and their growing population with the new strains of wheat, and Asia with the new strains of rice. Countries that relied on food aid were able to become self-sustaining, or, in some cases, become a net exporter of food.
The Green Revolution also adapted and spread some earlier 'revolutions' around the world; the programs almost always emphasized the use of added nitrogen fertilizer to increase crop yield, emphasized active irrigation, monoculture, and often pushed for increased mechanization. While all of these generally result in increased crop yields, they were not always applied judiciously, with many farmers taking a 'more is better' approach. This often resulted in both too much fertilizer on many fields, and too much water runoff spreading this fertilizer into local waterways and wells. Having fewer different types of crops in the farmers' fields meant that a flood or drought could wipe out the entire crop, resulting in worse famines during a string of bad years.
The Green Revolution also proved to be more limited that many proponents had predicted; while the new, hardy strains of crop were doing great in areas with large tracts of land with good growing soil and an established infrastructure, much of Africa does not have good growing soil, does not have well-developed infrastructure, does not have a long history of monoculture, and has many growing conditions outside the envelope of many of the crops produced by the Green Revolution. Borlaug spent much of his last two decades trying to bring the Green Revolution to Africa with very limited success.
The Green Revolution has allowed more people to exist, and to bring wealth and health to some parts of the world that people were seeing little hope for. It also encouraged some bad habits, which we are still trying to walk back, and has proven to be the wrong tool for some jobs.