@mentions might be new to you. In Teams, it's used to get the specific attention of a person, team or channel. It's most often used in a chat.
In a chat type, "@" and a name, team or channel.
In your activity feed, you can filter your @mentions to sort When you have been mentioned. This allows you to filter through the more important to-do's in your activity feed and filter out general activity.
You can bookmark a message or content to save it to easily find later.
You can mark chats or items as unread. This might seem obvious, but this can be used to remind yourself to come back to reply or read something later and not let it get lost.
You can see which items are unread, because they appear in bold in your chat window.
In activity, you can also filter for unread items.
DND stands for Do Not Disturb. You can use this to change your status so you can work heads down on a project. This will prevent pop-ups and other distractions as well as notify people you don't want to be bothered.