Add-ons allow the user to extend the functionality around Home Assistant by installing additional applications.
This can be running an application that Home Assistant can integrate with (like an MQTT broker) or to share the configuration via Samba for easy editing from other computers. Add-ons can be configured via the Supervisor panel in Home Assistant.
Add-ons for Home Assistant are freely available in the add-on store.
To install add-ons, navigate to the Supervisor panel in your Home Assistant frontend, and click on the "Add-on store" tab. All add-ons, including their documentation, are available right from the store. Some advanced add-ons will only be visible after you opt-in to "Advanced Mode" which can be changed on your user profile page. Click on an add-on you are interested in, to read the documentation or to install the add-on.
Add-ons are only available if you've used the Home Assistant Operating System or Home Assistant Supervised installation method. If you installed Home Assistant using any other method then you cannot use add-ons. Often you can achieve the same manually, refer to the documentation by the vendor of the application you'd like to install.
Check the Home Assistant forums for add-on repositories managed by the community.