Tails is “The Amnesiac Incognito Live System” and is an operating system designed for privacy and anonymity. You can install it on your computer, run it from a flash drive, or put it in a virtual machine. Like a tent, you can carry Tails with you and use it anywhere. To use Tails, shut down the computer and start on your Tails USB stick instead of starting on Windows, macOS, or Linux. You can temporarily turn your own computer into a secure machine. You can also stay safe while using the computer of somebody else.
Link: How Tails works