Keychain is a password management system for Mac. Dropbox requires access to your Keychain to help verify your account and to provide another layer of security over the Dropbox desktop app preferences. To grant Keychain access to Dropbox, click Allow or Always Allow when prompted. Always Allow is recommended for seamless access. Clicking Deny will result in a "Couldn't start Dropbox" error message. For security reasons, Dropbox will not start without access to your Keychain.