Your back is the part of your body that is behind you, between your bottom and your neck. The bone running down the centre of your back is called your spine.
A backpack is a type of bag that is designed to be worn on your back. Backpacks have two straps that go over your shoulders, and sometimes an extra strap that goes around your waist.
A jack is a device for lifting heavy objects off the ground by pushing them up from underneath. Jacks are used to raise things a short distance only. They are commonly used to lift one corner of a car off the ground while you change a tyre.
Jack is also a variation of the name John.
You pack a suitcase when you go away on vacation. Packing something means putting things inside it until it is full. You can pack a suitcase, boxes or even pack a wardrobe full of things.