What is Qubes OS?
Qubes OS is another security, privacy, and anonymity-focused operating system. The developers designed it with the belief that, currently, there's no tool or software that can completely protect us from zero-day attacks (attacks that exploit software vulnerabilities, which have, so far, not been discovered by the software vendor, and hence they have no patches or fixes yet, and can be exploited by hackers) and sophisticated hackers, who use tools and attacks that can bypass most, if not all, of an organization's defenses, be it firewall, antivirus, or another tool.
So, they came up with a great idea: compartmentalization, separating what we do on the computer into securely isolated compartments, which the developers named qubes.
Applications' hardware and sessions can be separated into their own secure qube, providing strict separation from the other qubes, and, this way, if the user surfs to a compromised website, and the user's browser is infected by malware, no other part...