Like other WhatsApp, Signal, and other messaging apps, Facebook Messenger supports end-to-end encryption. However, encrypted chats have lagged behind normal Messenger chats in features and availability, which the service is now improving.
Right now, you can create an end-to-end encrypted conversation with someone by creating a "secret" conversation, but the app has been slowly pushing its otherwise plaintext regular chats into the encrypted side as well. Now, though, this is perhaps the app's most considerable effort in that direction. The company is expanding testing of "default" end-to-end encrypted conversations, and you might already see a number of your chats get encrypted on their own. It won't happen to everyone at the same time, but it's happening eventually.
End-to-end encrypted chats are also gaining support for chat themes, reactions, active status, and link previews, and Android users can also see chat bubbles for these conversations. In short, they'll now work just like how current, unencrypted chats work. The end goal for this is to eventually make all, or most, Messenger chats end-to-end encrypted however possible.
The problem with end-to-end encryption is that it has historically proved tricky to do in a multi-device manner, and while messaging apps eventually began figuring it out, it's been a long time coming. The secret conversations currently supported on Messenger have historically not been accessible from multiple devices, nor from the desktop web version of Facebook, but rather only from the device they were created from. In the face of a wider encryption rollout, we should assume Facebook has something figured out for this. We'll see how this pans out over the course of this year.
Source: Meta