A telescope is a instrument which collects light and magnifies objects, making them look much closer. Telescopes do this by using curved mirrors or lenses to change the path of the light as it moves through the instrument. The way we move light around in this way is called optics.
They help astronomers do this by collecting more light than the eye can. This means that we can use telescopes to see much fainter objects than is possible with the "naked eye".
They can also let astronomers see more detail, although the atmosphere can make this more difficult.
In the past, telescopes were designed to let the astronomer look through them, but now all professional telescopes have special instruments on them.
Nowadays, we can also make telescopes that can detect kinds of light that the eye cannot see at all such as radio waves or X-rays. Sometimes these telescopes have to be made into satellites, or space telescopes, and put up above the atmosphere.