A more thorough explanation:

Definition: The Festo Doctrine is a rule in patent law that states that voluntarily narrowing a patent claim may result in prosecution-history estoppel. This means that if a patent holder narrows their claim during the patent application process, they may be limited in their ability to enforce the patent against infringers who use similar but not identical technology.

Example: Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co. is a case that illustrates the Festo Doctrine. In this case, Festo had a patent for a device used in manufacturing. During the patent application process, Festo narrowed the scope of their patent claim. Later, when Festo sued a competitor for patent infringement, the court found that the narrowed claim created prosecution-history estoppel, which limited Festo's ability to enforce their patent.

Another example: Imagine a company has a patent for a new type of smartphone. During the patent application process, the company narrows their claim to only cover smartphones with a specific type of camera. If a competitor creates a smartphone with a different type of camera, the company may not be able to enforce their patent against the competitor because of the Festo Doctrine.