The communications tracking functionality tracks all communications that you're exchanging with candidates. For example emails sent to candidates for requesting updated resumes, notifying a new job requirement etc.
The Communications module maintains & tracks all candidate communication in a central place. It provides a pre-defined view for easy and quick access to the relevant communications.
You can also use the Communications History icon shown next to candidates to jump to the view showing all communications (email, SMS etc.) that you've exchanged with a candidate.
Additionally you can perform common email functions such as sending/receiving email, managing your email folders etc. also via the Communications module.