It is the keyboard focus button. Super handy! "The Keyboard Focus in Pro Tools determines how the alpha keys function. Depending on which Keyboard Focus is enabled, you can use the keys on your alpha keyboard to select regions in the Audio or MIDI Regions List, enable or disable groups, or perform an edit or play command. An electronic PDF listing of keyboard shortcuts is available in Pro Tools. Choose Help > Keyboard Shortcuts... See page 30 of the Pro Tools Reference Guide for information.