When enabled, an Activity tab will be displayed on each record detail page where your users can leave comments about the record and react to comments from others:
Go to Setup Home - App Settings and check the collaboration to turn the feature on.
The feature is then enabled per-record type. From a record detail page, enter Edit mode by clicking the pencil button in the top right-hand corner:
In the Layout bar on the left, toggle the Enable Comments option:
That's it! The Activity tab will now be available when viewing records of this type.
There is now a notification icon in the menu bar that will signal when someone has reacted to your post, added a post to a record you are following (see Following, below), or mentioned you in a post.
Users can "mention" each other in comments by typing the @ symbol and the first letter of the recipients name. This will trigger a notification to the mentioned user.
You can opt to receive notifications when anyone posts on a particular record by toggling the Follow option to the right of the Activity tab: