The Plugin Manager allows you to install Minecraft Plugins without having to go to another site to download them! (Unless they are paid). You must be on a supported version of minecraft that uses plugins (Most likely all of them, unless its Vanilla).
When installing a new plugin it will automatically add the .jar file into the '/plugin' folder meaning all you have to do is restart your server!
When on the Plugin Manager page, click or search for the plugin you are looking for then click on "Install" to install the latest version or click on "Versions" to choose your own version.
It's highly recommended to use the latest versions of the plugin to make sure you have the latest bug fixes.
When you click on "Versions" on the plugin you are looking to install, you will get a pop up to choose the version you want!
To check if the plugin has installed properly, head over to your 'File Manager' and go in the 'Plugins' folder.
You will see the .jar of the plugin put in there with an icon of where it's sourced from next to it
