What happens when you bind 'this' on an Ember function?

I'm trying to better understand this Firebase authenticator for Ember SimpleAuth:

import Ember from 'ember';


export default Ember.Controller.extend({


actions: {
    login: function() {
        this.get('session').authenticate('authenticator:firebase', {
            'email': this.get('email'),
            'password': this.get('password')
        }).then(function() {
            this.transitionToRoute('index');
        }.bind(this));
    },
    logout: function() {
        this.get('session').invalidate().then(function() {
            this.transitionToRoute('login');
        }.bind(this));
    }
}

});

Would someone please explain what ".bind(this)" is doing and how exactly bind is working in this particular instance?