What is a shifting agriculture?

In poor developing countries, some farmers grow only enough food to feed their families, with a little left over to barter or sell for such things as clothes. This is called subsistence farming. In tropical countries, subsistence farmers clear plots in the forests or savanna and farm them for a few years. When the soil begins to become infertile and crop yields fall, they move on and clear new plots. This simple form of farming is called shifting agriculture.