Access this feature at:
Tools → Sandbox
The Sandbox allows you safely run an application or browse the web in an environment isolated from the rest of your system. This is useful when you want to test a suspicious application, or to safely visit a website potentially containing malware by running your web browser in the Sandbox.
To start an application in the Sandbox:
By default, the option is also available outside of Avast when you right-click on an application and select Run in sandbox or Always run in sandbox from the context menu.
To end the Sandbox session for that application, you can simply close the window. Or, you can terminate the process in Tools → Sandbox.
You can set preferences for the sandbox in:
Settings → Tools → Sandbox → Customize