Animator's HUD or AnimHUD for short is a viewport-integrated UI for major tools from AnimationToolbox menu. It gives animator quick and convenient access to most commonly used tools like Tweeener, IKFK Switcher, space switching tools, euler filter and more - while taking as little space on the screen as possible.
AnimHUD ui is semi-transparent and floats over the viewport, auto-hiding on playback, so it's only there when it's needed.
It is available from the main menu - BroTools - Animation Toolbox - AnimHUD.
For detailed documentation about each individual tool available in AnimHUD, please, refer to the main AnimationToolbox documentation page