Windows 10 Build 10125 is an unreleased and unleaked build of Windows 10 which includes a number of new changes and features over the last public preview build which is build 10122. Build 10125 includes new icons, improved customization to Start and a back button for tablet mode.
The site also mentions separately that this 10125 build adds support for Microsoft Hello, which is Windows 10's new security feature. Among other things, it will let users sign into the OS with their eyes, fingerprint or face if their PC has the right hardware. WinBeta did not have a PC that could actually run Windows Hello's features but it does show off the set-up process.