Chromebooks are multi-user, cloud based, computers that can be managed remotely. Looking through the Chrome Web Store there are cloud variants for most things you would do on a desktop. The one thing that has been missing is screencasting.
Screencastify is an app that records your tab. If you have used Google Hangouts or a Chromecast it will feel familiar. Its functionality is simple; choose your resolution, and audio source (mic or tab). With audio from tab it records the sound coming out of your speakers, and mic should be self explanatory.
When you have finished recording you can edit the name, delete, download, or upload to YouTube. That's it, yes it is that simple. Here is a video to prove it.