This is a special lens which can zoom in on the back surface of your eye (the retina).
The retina contains many cells which capture the image of the world. The optometrist scans the retina to make sure all the cells are healthy.
The ophthalmoscope also examines the optic nerve, commonly known as the blind spot. This is the nerve which transfers the signals from the retina to the brain allowing you to see.
The ophthalmoscope can detect many problems with the eye, including a disease called glaucoma, and also problems with the rest of your body such as diabetes and raised blood pressure.