Notification when something happens

Do someone have any idea about how to display a notification when for example a user is sending a message to another user in a Meteor system ?

Read more about the reactive variables or dictionary. Or the reactivness as a technology.

Do you mean a real push message or a message on screen?

For push message search: raix push that’s a meteor package for that.

For on screen what you basically could do:
Insert message in collection, set read = false.
When client sees the message update the message to set read= true.