Where does "this.get('filter')" comes from?

I've been following tutorials from Embers website and at certain point they do:

let filterInputValue = this.get('value');
let filterAction = this.get('filter');
filterAction(filterInputValue).then((filterResults) => this.set('results', filterResults));

And as far as I know this.get('filter') is the same thing as doing this.filter or this['filter'], right? I've been looking to their documentation about Component and its base object, since attributes/functions can be inherited, to find out about this filter but was unable to find it.

So, where does filter comes from?