Strictly speaking, a Go interface type defines the behavior of other types by specifying a set of methods that need to be implemented. For a type to satisfy an interface, it needs to implement all the methods required by that interface, which are usually not too many.
Putting it simply, interfaces are abstract types that define a set of functions that need to be implemented so that a type can be considered an instance of the interface. When this happens, we say that the type satisfies this interface. So, an interface is two things: a set of methods and a type, and it is used to define the behavior of other types.
The biggest advantage you get from having and using an interface is that you can pass a variable of a type that implements that particular interface to any function that expects a parameter of that specific interface. Without that amazing capability, interfaces...