Plugins are like apps for your WordPress site which adds functionalities to your website. They allow you to add additional features like contact forms, editor, page builder etc to your wordpress website.
To install plugins, first you need to visit Plugins » Add New page in the WordPress admin area. Next, you need to search for the plugin you are looking for by typing in keywords (like editor, contact form, etc).
If you know the name of the plugin that you want to install, then you can search with the plugin name directly.