Flash is a custom behavior that is created directly from GDevelop - allowing an object to blink
GDevelop allows you to create new behaviors, using the same events as the ones that you're using to create the rules of your game.
Behaviors can be created for any kind of object, and you can create custom actions and conditions associated to your new behavior.
An action is exposed by the behavior, and can be used like any other action in the events
Behaviors, as well as extensions, can be shared between projects. You can export and import them in a few clicks. If your behavior is generic and reusable in multiple games, you can even submit it to be integrated in the growing list of custom behaviors shared by the community.