The person subscribed gets an email once an ad, matching its search, is submitted.
You can subscribe on different "levels", depending on the search query, so if you subscribed in Cars category, you'll get an email everytime a new ad is posted into Cars.
While if you refine search by keyword "Honda", city "Sao Paolo" and max price "15000", you'll get an email once an ad for Honda is posted, with location Sao Paolo and price lower than 15000.
If a person is an user of your website, it can subscribe inside user panel (Alerts), otherwise the unsubscribe link is always inside alert email received.