Signal is a private messenger app, and it’s excellent.
It’s basically the antidote to the extensive network of Facebook and Google connected services that trade privacy for free web products. Facebook I hate, and don’t use, but I use all kinds of Google products and accept the trade off. Signal created the encryption, and licensed it to the largest messengers (Google, Facebook). Signal is nice because it’s free, it’s secure and it allows for a refuge of privacy by comparison.
The only thing Signal knows is whether or not your phone number has ever been connected, which is reasonable. You can use it to message anyone else who has the app in a way that is end-to-end encrypted. It’s funded primarily by the founder of WhatsApp, Brian Acton, as a non-profit. It’s also almost entirely open source, so you can see all the code online. It’s fast and relatively well used considering it is a niche app for protecting your privacy while messaging.
The way it works is pretty interesting. When you send a message, the only person who can see the message is the recipient - whatever is in the message cannot be viewed by Signal or anyone else. You make some trade offs for it, like no search and local-only back-ups, but that is a small downside. You can use it for sharing files, voice messages, even encrypted phone calls. Generally speaking if you care about privacy, it is the best way to accomplish it with low friction.
I suspect in the future that having privacy all the time won’t be practical. At the same time, constantly having multiple parties able to access your communication is oppressive and in many cases can be dangerous and in violation of fundamental human rights. Being able to use private messenger is a practical way of standing up for keeping those rights intact, without being too extreme. It’s good to be able to switch between them, and a reasonable compromise for most.
So in the words of Snowden: use Signal.