This is a mode for creating your own screencasts, to be displayed inside Emacs.
The file is self-documenting just load it, and type
Most screencasts are based on the info-page or describe-mode.
A reasonable question is: why not read the info-page, which contains more details, instead?
I have three answers:
If videos are superior in some way, why not just create a regular video displaying someone doing the things then?
I have three answers again:
A list of screencasts is available at ScreencastSources
You can save the screencast as a movie for some video service (you-tube etc.) using ScreencastRecord.
The recorded screencasts are listed at ScreencastRecordings.
EmacsScreencasts contains a list of screencasts about Emacs made without screencast-mode.