The crop usage icon shows how much crop you have available and how much you are using.
As with all resources, crop is stored in individual villages. Each village's granary can hold a certain amount of crop (depending on its level). Each village's crop fields produce a certain amount of crop per hour (depending on their levels), and the buildings and troops in the village consume a certain amount of crop per hour.
In the image above, the granary statistics are circled in green. The granary in this village can store a maximum of 45700 units of crop, and currently contains 3219 units.
The crop usage is circled in red. The village's crop fields (and possibly grain mill and bakery) produce 1001 units of crop per hour. The village's buildings and troops consume 278 units of crop per hour.
In this example, the village is consuming more crop (wheat) than it is producing. This means it must receive crop from outside sources (merchants, or raids) regularly in order to avoid troop starvation.
Ensure you plant enough crop fields to keep supplies up. You should plan to have a grain mill and a bakery in your capital because of their multiplier effect on your crop fields, which can go to higher levels in the capital. At some stage you may want those facilities elsewhere too, especially if you have a 9-cropper or a 15-cropper village.