Here's an example of tweening the position of a label on the X axis. The only trick is in knowing the syntax of accessing a single component of the rect_position
object...
func _ready():
var tween = Tween.new();
tween.interpolate_property($Label, "rect_position:x", 100, 200, 3, Tween.TRANS_LINEAR, Tween.EASE_IN_OUT)
add_child(tween)
tween.start()