This is the input highlight text effect as seen on Tripadvisor search, as you enter the input the underscore highlight effect level with the end of the text. Designed by Petr Gazarov.
If you are having trouble with the pen, try the archived copy on GitHub
Tripadvisor Input Highlight Snippet
See the Pen Tripadvisor input highlight by Petr Gazarov (@petrgazarov) on CodePen.0