A trigger is a mechanism that gets executed automatically from the DB every time the trigger condition is met.
In your case the trigger definition is BEFORE DELETE ON person
. So every time before a delete on the table person
occurs, the DB executes your trigger.
A Stored Procedure on the other hand is a function that you can call every time you like manually.
And another existing mechanism are Events. You can execute them on a time schedule you define i.e. every hour, automatically.