Crop rotation is planting a different crop in a field than what was planted the previous year. For example, planting corn the first year and soybeans the next year is a corn/soybean rotation. Planting corn the first year, soybeans the second year and wheat the third year is a corn/soybean/wheat rotation.
Farmers rotate their field primarily to break disease cycles. Planting a continuous crop (soybeans year after year after year) tends to allow the field to build up with the diseases (and insects) that affect soybeans. Adding in an additional crop will tend to eliminate the diseases that affect soybeans, because you remove the host (soybeans) for at least one year. The more crops that are included in a rotation, the more time there is for the disease to die out, and the less likely that the crop will be infected.
A secondary benefit is when a legume (like soybeans or alfalfa) is included, nitrogen is provided for cereal crops like corn and wheat.