A cropper refers to a village with a high number of cropland, usually a 15-cropper (which has 15 crop lands) or 9-cropper (which has 9 crop lands). It is rarely, if ever, used to refer to 7-cropper or the normal 6-cropper.
These villages are particularly valuable as crop is required to support large armies, reaching an apex in the end game phase. In the end game, most players will support large armies, and crop being the only resource whose production decreases due to upkeep, leading to a relative scarcity.
Crop also has highest production bonus. Each village can receive up to 50% crop production bonus from building and up to maximum 150% crop production bonus from oases. Those numbers for other resource types (wood, clay, iron) are 25% and 75% respectively. Hence, villages with large numer of croplands will produce most resources.
Therefore, to maximize crop production, most players select their capital at a cropper (so that their 9 or 15 croplands can be upgraded beyond level 10). Croplands with a nearby high crop oasis can push their crop production even further; the maximum enhancement from crop oases is 150%. 15-croppers with such high crop oasis enhancements (100-150%) are very rare and heavily contested. Conquering such croppers is a common reason for war.