The feature, which would form part of Twitter's Direct Messages, is currently titled "Secret Conversation," with users able to view your, and their, encryption keys. That's pretty much all that's known about the service right now, but it's likely that Twitter is thinking about how better protect its users. Not to mention, as the report points out, such a feature was initially requested by Edward Snowden at the end of 2016.
Twitter is coming under the same scrutiny as many other social platforms, all of which need to demonstrate that they can be trusted with your personal data (oops!). The service, famously, played a part in fomenting the uprising that would come to be known as the Arab Spring. But has since found itself as a vessel for nation state propaganda -- a problem that it's working hard to resolve as it finally reaches profitability.