what this syntax in typescript interface means?

Here's an annotated version:

// There's a namespace named Patterns.Interfaces
module Patterns.Interfaces {
    // It has an interface named IObservable
    // It is visible outside this block ('export')
    export interface IObservable {
        // An IObservable has a function called RegisterObserver.
        // It takes one argument named 'Observer'.
        // 'Observer' is of type Pattern.Interfaces.IObserver.
        // You must pass this argument.
        // Its return type is unspecified, so is assumed to be 'any'
        RegisterObserver(Observer: Patterns.Interfaces.IObserver);//Patterns.Interfaces.IObserver type?
        // Same as above
        RemoveObserver(Observer: Patterns.Interfaces.IObserver);
        // An IObservable has a function called NotifyObservers.
        // It is called with no arguments.
        // Its return type is unspecified, so is assumed to be 'any'
        NotifyObservers();
    }
}