The Secret DM feature for Twitter was found within the Android application package by Jane Manchun Wong, and it includes "Encryption settings," details about what encrypted messages are, and Encryption keys to verify a secure conversation between you and the message's recipient. Wong also found that users will soon be able to quote tweets with GIFs and place a "Quality Filter" on search results.
"Secret conversations" is the same name used by Facebook Messenger's own end-to-end encrypted messaging feature, which completed a rollout in October 2016. At the time, Facebook said Messenger supports the Signal Protocol to encrypt its messages, the same cryptographic protocol that WhatsApp and Google Allo also use.
As TechCrunch pointed out, APKs typically include code for features that companies are testing or will be launching soon, but it's unclear when exactly Twitter's encrypted messaging update will debut.
Last week, Twitter found a bug that "stored passwords unmasked in an internal log," and although no one appeared to have accessed the plaintext passwords it advised all users to update their own passwords "out of an abundance of caution."