Let's say you manage a WordPress site. You chose, purchase, and install a pre-made theme. Say you added a few items you came across in the WordPress plugin directory to add some advanced features to the site. This is the awesomeness that is the WordPress ecosystem. It's relatively easy for anyone with light technical chops to get a website off the ground and wrangle together something powerful without having to build everything from scratch. It just works great and your website looks wonderful.
Until it doesn't.