self is used to refer to the current instance where you use it.
CONCAT(self.last_name, ", ", self.first_name)
This expression calculates the full name of a person, by getting the information from the fields
first_name, separating those using a comma and a space.
If the expression is attached to an instance of the ‘Person’ ontology,
self tells the system to look for the fields last and first name within that instance.
If there, it finds the field
last_name holds the value 'Johansdottir' and
first_name holds 'Britta', this expression has as output: Johansdottir, Britta