Wkipedia has an article Signal (software).
Signal is an encrypted instant messaging and voice calling application for Android and iOS. It uses the Internet to send one-to-one and group messages, which can include images and video messages, and make one-to-one voice calls.
In one of the last two posts, I believe one of the interviewees mentioned Signal as an encryption app. I think it was either Seymour Hersh or Glen Greenwald. In any case, I thought it might be good to provide the link to find out more about what they were talking about.