The messaging app was updated to include a microphone icon where the send button used to be. That allows the user to record a voice message, completing the circle of sendable media: text, voice, video, and photo.
This was already present a while ago on Facebook Messenger since its last major revamp in late October 2013. Save for one feature, the Messages update was truly just a game of catchup: sending your location. While tagging your messages with your current location has been a feature in Facebook Messenger since mid–2012, it’s been absent from Android’s stock messages app.