How should I understand Go interfaces?

For a rough runtime model, an interface value stores type-related information along with a reference to the underlying value. This lets Go keep track of the concrete type and dispatch the appropriate method implementation. The exact representation depends on whether the interface is empty and on the value involved, so it’s better to learn the behavior and method-set rules first rather than relying too heavily on a simplified two-word layout. In practice, check whether the required method signatures match exactly; the compiler handles the rest.

Think of an interface as a set of capabilities rather than a concrete object. It lists method signatures, and any type that implements all of those methods automatically satisfies the interface. Code can then work with the interface without needing to know the exact underlying type. For example, anything with a Read method can satisfy an interface containing Read.

The official Go Tour is a great place to start, especially the sections on methods and interfaces. The key idea is that Go uses implicit implementation: you don’t declare that a type implements an interface. If its method set matches the interface, the compiler considers it a match.