To provide users stronger authentication protection, Dropbox adds Universal 2nd Factor (U2F) security keys as an additional method for two-step verification.
It’s an easy and secure way to use these keys for accessing Dropbox services or singing into dropbox.com. To add an extra to your account, two step verification is an highly recommended security feature and it’s simple though. You jut have to sign in to your account with your password and then insert the key into USB port when prompted to do so.
How do I use it ?
Once you have the key, sign in to your account and then go to account settings>security>add next to Security Keys. Currently it’s only supported for dropbox.com using the Chrome web browser.
For more details head over to the below source.