Hello @kubbyelizabeth,
For this application, the authentication and security layer is handled by SteemConnect. The application Fast Reply never handles any of your keys. Instead, it receives a token (which remains in your browser) that has a given level of priviledges.
Since the application only receives the rights to vote/comment and follow/unfollow/mute users. That is the maximal risk exposure you take. Of course, you can -- at any time -- decide to revoke the access to the app using the SteemConnect dashboard (available in the top-left menu of the app).