Disclaimer: The following are glossaries I've written to simplify sharing what the Signal application is all about with friends or others. I am not associated with Signal and/or Open Whisper Systems in any way.
Signal is a free and open source software application for Android, iOS, and Desktop that employs end-to-end encryption (E2EE), allowing users to send end-to-end encrypted group, text, picture, video messages or other attachments, and have encrypted phone conversations between Signal users. Although Signal uses telephone numbers as contacts, encrypted calls and messages use your data connection so both parties to the conversation must have internet access on their mobile devices.