Add-ins are already available on the desktop, but now you can access third party apps like Evernote, GIPHY, Nimble, Trello and Smartsheet on your phone without leaving Outlook. With these add-ins, you can translate an email in a foreign language, pin it to a Trello board, and embed a snarky GIF in your reply. Once the feature is turned on, you'll see an icon in the email header. Tapping it will launch a quick menu with all of the add-ins you've enabled.
Microsoft has been steadily working to revamp Outlook mobile since its acquisition of the popular calendar app Sunrise in 2015. The company eventually killed Sunrise and replaced it with Outlook, but not before it cannibalized most of Sunrise's best features.