While working on your website, you might be building out a top secret feature that you don't want anyone to access except for certain Team Members or external stakeholders. In these cases, some sort of protection mechanism is strongly encouraged.
For team accounts (Pro or Enterprise), Vercel offers Password Protection and Vercel Authentication for Preview and Production deployments.
Password Protection allows you to place your deployments behind a lock screen that only lets visitor's through after they've entered the password you've configured.
Vercel Authentication, like Password Protection, places your deployments behind a lock screen that only lets vistor's through if they are a member of your team and log in through their Vercel account.
By entrusting Vercel with the responsibility of protecting your sites, you free your team from the engineering time involved with building a custom solution, time involved with maintaining that solution, and you reduce the potential cost of things going wrong due to security breaches.
As a simple toggle in your project settings, Vercel's Deployment Protection facilities offer you a truly serverless way to protect your work while previewing, so you can share it with the world only when ready.