The Waterfall Chart shows the request-by-request loading behaviour of your webpage. It displays every single resource requested by your page, like scripts, media files, third-party resources, etc.
With a Waterfall Chart, you can see what requests are loaded and in which order. The duration and execution time of each request is also displayed, with colourful bars representing how long each one took to request, download, and/or execute.
The Waterfall Chart aids you in debugging performance issues, as well as, general page issues as a whole. You will find the Waterfall Chart on a separate Waterfall Tab on the GTmetrix Report.
Click here to learn more about how to read a Waterfall Chart.