The Activity app in Microsoft Teams is used to show a feed of activity that is relevant to you in Teams. It shows activity such as @ mentions, replies, reactions to your posts, and unread posts in channels that you are a member of.

It can be found on the left-hand side of the Teams application.

Here you will see the activity feed for your profile. If there is too much information to sort through, you can filter the feed down to show specific activity catergories.

Choosing a specific activity category will make it easier to find relevant notifications.