Skype for Business is part of the Mobile Office mailbox.
Working with Skype for Business
When you install the complete Office 2016 package, the Skype for Business application is also included. With Skype for Business you can easily (video) chat, call or meet with your colleagues.
You can also install Skype for Business on all your mobile devices. Skype (for business) apps are available for all mobile platforms (Android, iOS and Windows Phone).
Also check out Microsoft's Skype for Business support pages: