Phantom types are a type of generic types in programming, where a type parameter is declared but not used directly within the definition of a data structure, function, or class. They're commonly used to enforce additional type safety without having to include actual data of that type in the data structure. This can help prevent certain types of errors at compile-time, offering a form of static typing benefits.

For more details, you can refer to this guide on generics that includes a section about phantom types: Move Basics Generics.