Plugins are third-party ‘apps’ that extend the functionality of your WordPress site. They should be used sparingly, as each one increases the load time of your site. There are both free and premium (paid-for) plugins.
Great care should be taken when installing plugins on WordPress. You should always make sure they are compatible with the version of WordPress you are using.
Often, plugin developers get behind in keeping their plugins up-to-date – this is one of the most common reasons WordPress sites get hacked.