If you have watched any of my how-to videos for the website or google classroom, you have seen a screencast. I'm also putting a sample below that's just 2 minutes. It is a video you make of your computer screen and optionally you can include your voice or even yourself in it. They can be used to model a difficult concept or just about anything. It is very easy to set up and even easier to use. Watch mine below for more information, or use the links to help with downloading the app onto your computer.