Setting up your head position is very important for the development of your swimming as setting this up correctly can quickly improve your body position too. It will enable you to hold your core posture (no arching your back) and allow a stronger arm movement under the water through a much improved rotational axis created by a stable head position.
Watch this video below to learn more about the drag components created by a poor head position, then compare this to a 'down' head position.
NB. The video does discuss the arm movement under the water but if you are just starting off in swim lessons- don't worry about this initially just get the head position right. The catch is developed in later lessons.