Mautic plugins are installable packages which can extend Mautic functionality or integrate 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.
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 button 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 plugins", clear your Mautic cache and try the Install/Upgrade plugins button again.