Plugins
Mautic plugins are installable packages which can extend Mautic functionality or connect it with another system. If you are interested in how to create a new Mautic plugin, read more about it in the developer documentation.
You can find the Plugins in the right admin menu.

Install plugins
If you are on a freshly installed Mautic instance, there is a chance that you don't have the default plugins installed yet. Click on the "Install/Upgrade plugins in the top right corner and all the plugins should appear."