Optics is a branch of physics which looks at how light behaves and how we can observe it. This usually involves using mirrors and lenses to magnify, focus and move the path of light so that it is easier for us to see. An every day example of this is the use of lenses in glasses worn by people who's eyes cannot focus light properly. The lenses in this case can change the point at which the light is focused to correct the eye, and allow our brains to see a focused image.
The main use of optics in astronomy is within telescopes, where we uses lenses or mirrors to magnify the light coming from distant objects in the Universe so that we can see them more clearly. We also use lenses and mirrors to focus the light and allow us to place cameras in place of our eyes so that we can get data from the observations.
When we trace out the path of light through lenses and mirrors, we often refer to this path as a ray diagram and it will look very different depending upon the type of telescope.