ECA stands for "Events - Conditions - Actions" and is a powerful, versatile, and user-friendly rules engine for Drupal 9+. The core module is a processor that validates and executes event-condition-action plugins. Integrated with graphical user interfaces like BPMN.iO, Camunda or other possible future modellers, ECA is a robust system for building conditionally triggered action sets.
Module
What's known as plugin or addon in other platforms, is what we call a module in the Drupal universe. Drupal core itself is a a collection of modules as well and the community has added and maintains tens of thousands of individual modules on top of that.