It's a method that comes from rxjs library which Angular is using internally.
If you can imagine yourself subscribing to a newsletter, every time there is a new newsletter, they will send it to your home (the method inside subscribe gets called).
That's what happens when you subscribing to a source of magazines ( which is called an Observable in rxjs library)
All the AJAX calls in Angular are using rxjs internally and in order to use any of them, you've got to use the method name, e.g get, and then call subscribe on it, because get returns and Observable.
Also, when writing this code <button (click)="doSomething()">, Angular is using Observables internally and subscribes you to that source of event, which in this case is a click event.
Back to our analogy of Observables and newsletter stores, after you've subscribed, as soon as and as long as there is a new magazine, they'll send it to you unless you go and unsubscribe from them for which you have to remember the subscription number or id, which in rxjs case it would be like :
let subscription = magazineStore.getMagazines().subscribe(
(newMagazine)=>{
console.log('newMagazine',newMagazine);
});
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myObservable.subscribe(
x => console.log('Observer got a next value: ' + x),
err => console.error('Observer got an error: ' + err),
() => console.log('Observer got a complete notification')
);