The feature is part of Editor, Microsoft Word's AI-powered writing assistant. Essentially, the tool takes things that Word users have been doing for years to create makeshift lists and notes -- or using other apps to do -- and turning it into something more uniform within the Word ecosystem.
If you type things like "todo" or "<insert something here>" in a Word document, Editor will recognize the text and create a list based on the items. It will also make recommendations based on other recent documents and files that you've been working with. If you type <insert graph here>, the AI will display recent charts you've worked with in a sidebar so you can insert the relevant one.
If you're working in a shared, collaborative Word document, you'll be able to @-mention colleagues to let them know what tasks they need to tackled. The tagged person will receive an email about the notification, and can take action on the document by replying directly to the email. So if there's a piece of information missing, they can include it in the reply email and it'll automatically be added into the Word file.
The new features will be available starting today as a preview to Mac for Office Insiders users on the Fast level, which is the group of users who get features first to provide feedback. Slow level Insiders, who get more stable versions of features before they are included in public releases, will be in line to get the feature later. A Windows version will be tested in the coming weeks. After all thet testing is complete the features will roll out to all existing Office 365 users, with other Office apps scheduled to get the feature down the road.