A plug-in is a package of HTML, JavaScript, and CSS files for setting screens.
You can use plug-ins with kintone by importing them in kintone Administration and adding them to apps individually.

To be able to manage plug-ins, including the ability to import them in kintone Administration and check their usage status, you need to have kintone administrative permission.
You can also restrict the use of plug-ins by specifying which apps are allowed to use them.

If you are a Users & System Administrator, it is also possible to download a list in CSV format of the plug-ins that have been imported in kintone.

For details on how to add plug-ins to apps, refer to the following page.
Adding/Deleting Plug-Ins (App Settings)