A screencast is a video recording of your screen. This is often used to show support agents exactly what's happening when an error occurs, usually while recording a HAR file to get a complete view of a problem. Having both a screencast and a HAR file makes solving issues happen more quickly and easily.
With Chrome, recording a screencast is easy - just install the Screencastify extension for Chrome. After it's installed, click the extension and then click "record" to record a screencast. You can even record your entire desktop if you're having an issue with a desktop application like Google Drive File Stream.
As a bonus, Screencastify can automatically upload your screencasts to your Google Drive account to make them easy to share with support agents. Just press "settings" and select "Google Drive". The only caveat is that the free version will watermark the video and limit recordings to ten minutes.
If your organization doesn't permit installing extensions, check to see if you have access to another screen recording software program.
If you're unable to create a screen recording, provide a list of detailed steps reproducing the error and screenshots for each step.