Feedback inhibition is a process by which metabolic pathways are regulated. It's an elegant method for an organism to conserve energy while still disallowing a runaway metabolic pathway to wreak havoc on the delicate balance of chemical reactions that make up metabolism.

It works like this: the final product in a pathway will inhibit an earlier step in the pathway. It's like an artificial enzyme inhibitor, the presence of the final product in a certain key concentration or greater slows down any further pathway reactions. Once the concentration drops below that key point, the reaction speeds up again to replenish the product. This type of inhibition is usually only found in linear pathways. An ASCII diagram:

A --> B --> C --> D --> ... --> Final Product
       X                           |