If your website is based on a content management system like WordPress, Drupal, Joomla, etc., you’ve no doubt been intrigued by a plugin that adds functionality to these systems. Stop. Don’t press install!
If you want to try out new plugins to see if they meet your needs, it is really, really important to do so on a separate installation of your CMS that can be deleted and rebuilt from time to time. We call this a sandbox and it is the place for experimenting and testing things before we decide to use them on our live, public websites.
Let’s use WordPress as an example. Plugins extend the WordPress database. They add taxonomies. They install code that may or may not have vulnerabilities. They sometimes prove incompatible with other plugins or themes. None of these are things you want to have littering or impacting your WordPress installation forever more.