Polyculture is the cultivation of different plants in the same field. This might mean merely different varieties of the same crop, or entirely different crops. It was the standard practice of farmers since farming began until the advent of monoculture in the modern age.
Nature, as anyone can tell by looking at uncultivated land, does not grow plants in neat units. Not only are there hundreds of different varieties of many plant species, but they grow next to plants of entirely different species. This situation has come about because uneven ground conditions might favour one plant in one square meter and another in the next; and because healthy soil requires a mix of plants to maintain a nutrient balance. Diversity also protects the plants against parasites. Monoculture attempts to eliminate this untidy diversity altogether, but polyculture merely aims to simplify it and exclude plants which are useless to humans.
Polyculture is useful in peasant societies because it provides for security against starvation. A farmer in southeast Asia might know of dozens of different varieties of rice - their cooking and storage properties, for instance - and cultivate many of them in the same field to provide himself with food security. As the varieties have different vulnerabilities to parasites and disease, a mixture ensures the whole crop can't be wiped out by one blight.
A field might contain numerous varieties of rice and also coffee plants, bushes which produce large, luxurious leaves for wrapping items with, and fruit trees. This wouldn't be a neat, ordered field like the ones we're used to in mechanized farming, but a large, apparently uneven surface. Polyculture would allow the farmer to maximize what he can grow on imperfect ground, and this is why it remains prevalent in many parts of the developing world.