In Drupal 7 if you wanted to tweak the functionality of an existing function then an alter hook was your best friend, but in Drupal 8 it's "all change!"

With the introduction of Symphony, your new BFF is an Event Subscriber. Alter hooks still exist, but it is recommended that we all move towards Events to fall in line with Symphony.

If you are interested in the comparison between alter hooks and events then I recommend this article from PreviousNext.