The gnomon is a control used to adjust position and orientation of coordinate frames and transform objects in the viewport.
One dimensional translation
Move the gnomon along the X, Y, or Z axis by clicking the axis and moving the cursor.
Moving the gnomon along the X axis:
2D translation
Move the gnomon along the XY, YZ, or XZ plane by clicking the plane (in gray) and moving the cursor.
Moving the gnomon on the YZ plane:
3D translations
Move the entire gnomon freely along the view plane by clicking the origin and moving the cursor.
An example of moving the gnomon origin:
2D rotations
pt 1: move
3D rotations