For example, radishes that mature from seed in 25-30 days can be grown between rows of most vegetables, and harvested long before the main crop matures. Or, a catch crop can be planted between the spring harvest and fall planting of some crops.
Catch cropping is a type of succession planting. It makes more efficient use of growing space.
Catch crops are also crops that are sown to prevent minerals being flushed away from the soil. By using catch crops, such as grain (millet, ...) one can keep certain minerals not attached to the humous-clay connection (such as carbon (C) and other positively charged elements) in the soil for (many) years.