Plugins are optional parts of pretix that can be installed to extend the available functionality and that can be turned on or off completely for every event. For your event, a number of plugins might be active already, but you can unlock even more functionality by going to “Settings” → “Plugins” and enable more of them, if you need.
For each plugin, you will find a short description as well as an Enable/Disable button. The pretix website has an overview of available plugins and more details of them. If you are on the pretix.eu hosted service, look for the “pretix Hosted” badge in the plugin list to learn which ones are supported there.