You can simply download Explain Everything from the Google Play Store.
However, not all Chromebooks support Google Play Store. Before starting, please check if your device supports Android Apps.
How to enable Android apps on your device
First of all, please make sure you are running the latest version of Chrome OS. Your Chromebook must run Chrome OS version 53 and up.
1. Click on your account photo.
2. Click Settings, and scroll down to the Google Play Store section and click on TURN ON.
3. A new window will pop up. Click on Get Started and you’ll be asked to agree to the Terms of Service. When you’ve read and agreed, click Agree.