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.
If you are trying to install a new plugin that did not come with the original Mautic installation files and it does not appear after you click on "Install/Upgrade" clear your Mautic cache and try "Install\Upgrade" again.