This is a nifty little animation to show quotes from users or something similar on web pages where you have multiple tabs kinda like an accordion menu or even a navigation menu and when you click on an item to activate it the underline turns into a speech bubble. I really love the minimalist approach in this design, it can work well on comic book themed websites.
If you are having trouble with the pen, try the archived copy on GitHub
Accordion to Speech Bubble Snippet
See the Pen Animated Speech Bubble Nav by schadeck (@schadeck) on CodePen.0