'Signal for Android' is an innovative communication app that is being offered by a company by the name of Open Whisper Systems.
This app was only recently launched, and is planning to make it more safe and secure for people to exchange sensitive communications without having them come into the possession of prying eyes, nefarious characters and other undesirable kinds of people.
This particular app actually works by combining two of Open Whisper Systems' existing products, namely RedPhone and TextSecure, into a single client. This allows you to communicate by using a fully encrypted connection without having to switch apps all the time.
The Signal for Android app is endorsed by none other than National Security Agency whistleblower Edward Snowden, so you know it probably works really well.