Signal is a free, open-source encrypted messaging app available on iOS and Android devices.
Signal uses end-to-end encryption on text messages and calls made to other Signal app users, which means that the communications are only able to be read by those users in the Signal conversation.
Using your data/wifi connection, Signal users can send text, images, video messages and make audio calls either to individuals or to groups created within the Signal app.
Why would I want to use Signal?
In some contexts communications between Christians are attempted to be monitored by others. Signal allows to you have communications with contacts also using Signal which are encrypted which would enable you to have those secure conversations and messages.