If you have had a neck dissection, you may struggle with movement of the arm on the side of the surgery. The trapezoid muscle may be traumatised and so the anterior deltoid takes over and may be doing work to compensate. The exercises get that muscle working again enabling you to lift the arm and cross the arm over your chest. Start slowly and as you get stronger use a weight starting with half a kilo or a can of soup.
Good luck, these really helped me.
Stay positive!