In general terms, addons are components that change the behavior of a core component. As a developer, you might be familiar with using browser extensions to support development tasks. In such a scenario, the web browser acts as the core component and the extensions act as the addons that install on top of the web browser. The addons in WordPress follows the same concept. However, WordPress addons itself are plugins that change the behavior of a main plugin. Unlike browser extensions, addons in WordPress don't install on top of a plugin. Instead, they act as separate addon plugins.
Usually, addons are used to add new features to the core component. However, WordPress addons are developed to add, change, or remove features of a core component. Let's take a look at some of the popular plugins with a large addons base: