I think you may be looking at the auto Sandbox which is different to the on demand sandbox which can sandbox your browser, open settings and click on the ? at the bottom for the help guide.
You should always try to surf the internet safely as no Anti-Virus can 100% detect and protect you all the time.
Keeping your system clean is automatic as avast is always scanning in realtime, you can add a quick or full scan set to start at a specific time once a week/fortnight for added piece of mind.