Event scheduling: The assignment to signal x does not happen instantly. Each of the values assigned to x contain an afterclause. The mechanism for delaying the new value is called scheduling an event. By assigning port x a new value, anevent was scheduled 0.5ns in the future that contains the new value forsignal x. when the event matures, signal receives a new value. Event is nothing but change on target signal which is to be updated.
Example: X<= a after 0.5ns when select=0 else
X<= b after 0.5ns