You download a plugin and while it almost does what you want it to do, with a bit of tweaking it would do exactly what you want it to do. Actions and filters allow you to modify how a plugin works without hacking away at the plugin itself, if that plugin’s author was kind enough to add them in.
This talk shows you what actions and filters are, how to use them, and a couple techniques on how to find the right action and filter to change what you want to change.
The full version of the talk from WordCamp Toronto 2015 is here, complete with some funny parts like a guy pitching his own plugin in the middle of the talk 🙂
or the edited 10 minute version: