Yes, that was the wrong term.
For notifications you should do the following:
Integrate Websockets (to get real time notifications, you can take a look at socket.io), if you don’t mind to get notifications only during refresh then use the Rest API. You can also use Rest API to get periodically new notifications, like every 30seconds.
Create a collection type called Notifications (with fields: Type [Enumeration: Mention, Reply], Relation with User [User has many Notifications], Read [Boolean])
When someone replies to the author’s post, then you create a new entry in notification with Relation to the user and with Type: Reply. Now you can use Websockets to emit a new event to the user, so it will get the new notification in realtime, or you can fetch them Periodically with RestAPI. Now you count all the user’s notifications with Read:false and display a badge with the count in frontend. When user clicks on a notifications you make a call to API and set Read: true, if the call was successful then you subtract -1 from counts in the frontend.
I hope that helps you a bit to understand what to do.