Great question. A telescope uses either mirrors or lenses (depending on the design) to gather light from a much larger area and focus it on your eye. It’s like how some people need glasses to see things that are far away – the lens does the work in focussing the light correctly for them to see it.
For telescopes, the bigger the lens or mirror, the more light it can collect, and then the further you can see. It’s just a question of getting the shape of the lenses and mirrors right, to make sure the light is focussed in the right place, and making sure that there’s no scratches or imperfections that mess it up.
There’s scientists who do loads of research into space taking part in this, so maybe they can give you more detail than I can! 🙂