You can start looking at the template because that is where the social media content is extracted from. The subject value is from the #messageSubject local variable, as illustrated:
<ion-item> <h2 #messageSubject>Ionic Developer</h2> <p>May 5, 2016</p> </ion-item>
In the preceding case, the subject is Ionic Developer because you will access messageSubject.innerText later on. messageSubject is just referencing your H2 DOM node.
Similarly, the body is from #messageBody, as shown here:
<ion-card-content> <p #messageBody>Wow Ionic 2 is so awesome. I gotta share this to other people.</p> </ion-card-content>
When the user clicks on the SHARE button, it will trigger the sendShare() method, as follows:
<button ion-button color="primary" ...