Dropbox announced a more secure way of accessing your files on its cloud storage service by offering U2F or Universal 2nd Factor.
U2F security keys adds another layer of protection when accessing your Dropbox account by implementing a two-step verification system. It’s similar to how Google utilizes it, Microsoft, and other companies that seem to care about your security.
How does it work with Dropbox? You type in your username and password like you normally would do but then you are prompted to insert a Dropbox USB security key, instead of having to type a six-digit code. Some other two-factor authentication utilizes an app on your phone or phone or text message to your phone and while these methods work, the DropBox method seems a bit more secure.
Currently the U2F is only supported for dropbox.com using Chrome but they have plans to get the feature working with other browsers as well.
More information: DropBox Blog