Interfaces in TypeScript allow you to define the shape of an object, acting as a contract that specifies the properties and their types that an object must have. By using interfaces, you can ensure that different parts of your code use objects consistently and correctly. An interface only describes the structure; it does not provide implementation. This makes interfaces a powerful tool for enforcing design and improving code maintainability, especially as your codebase grows.

Interfaces can be extended to build upon existing contracts, allowing you to add new properties or combine multiple interfaces. You use the extends keyword to create a new interface based on one or more existing ones. This helps you avoid repeating code and makes your types more flexible. You can also combine interfaces using intersection types with the & operator, creating a type that includes all properties from multiple interfaces.