The rotation variable contains the rotation angle in radians. rotation_degrees is the same value, but in degrees.
To convert from radians to degrees, you can use the rad2deg()
function.
You can also just change rotation_degrees without touching rotation.
For example, if you want to make your player face the mouse, you can use this:
var angle = (get_global_mouse_position()-get_node("..").position).angle()
rotation_degrees = rad2deg(angle)