I think they are called "Placeholders" - but they are the little codes that are used in email campaigns that personalize messages.
For example, in email newsletters, you can write Hello [name] or {name} -- and then the name of the person that you are sending the email to goes into the place of that "placeholder"
I think that this would be great option if we had this in heartbeat, to make notifications more personal, but especially for the messages that a person sees during the on-boarding process and also for courses.