Microsoft is testing a new feature for Skype: end-to-end encrypted chat conversations, secured using the industry standard Signal Protocol by Open Whisper Systems.
Private Conversations feature. The feature is made possible via Signal Protocol’s encryption, though it isn’t available to non-Insiders at this time. You can become a Skype Insider to get access here
It means the contents of messages can only be read by the sender and recipient. The encryption will cover chat, files, and audio messages, but not audio or video calls.