"Mentions" are something you'll be familiar with if you use communication tools like Slack or Confluence, or social media apps like Twitter or Instagram. You type the "@" symbol and then someone's name, and they get a message saying you've mentioned them in a message/post/article. Mentions have also turned up in the latest versions of Outlook, so let's take a look at what they do.
Microsoft's latest versions of Outlook are Outlook 2016, Outlook 365, and Outlook.com. If you're not using one of these versions, you won't have mentions until you upgrade.
Mentions work in Outlook in much the same way as they do everywhere else you've used them. Type an "@" symbol in the body of an email or event and then start typing a name and any matching contacts will appear for you to select.
Keep typing until the right name is displayed, or use the Up and Down arrows on your keyboard to select the right name from the list, and hit Return/Enter on your keyboard. The mention will be displayed, and the mentioned person will automatically be added to the "To" field.
And that's it, at least from the sender's point of view. You can mention as many people you want, and then send the email when you're ready.
As the receiver of the mail, a couple of things are different when you receive a mail in which someone has mentioned you. The default folder view in Outlook (in the versions we mentioned at the top of this article) now includes a "Mention" column.
When you receive a mail in which you've been mentioned, an "@" symbol will be visible in the Mention column.
You've also got an additional "Mentioned Mail" filter you can choose if you want to see just the messages where someone has mentioned you.
So, what's the point?
Well, in all honesty, not much, as far as we can see. The whole point of using a mention in Twitter or Slack is to bring someone into the conversation or alert them, but you're already doing that in Outlook by simply sending them the message.
If we had to guess, we'd say Microsoft is just laying some groundwork for the future---most likely for some kind of integration with Microsoft Teams. There are plenty of people who use Microsoft products at work, but not Slack, Twitter, or other social media. Microsoft is heavily pushing Teams for its enterprise customers, and---no surprise here---mentions feature heavily in Teams the same way they do in Slack.
It might be that Microsoft is just getting people used to the idea of mentions, but it's more likely that they're planning on some integration between Outlook and Teams (and probably server products like Exchange and SharePoint) that we're just not seeing yet. Time will tell.