The Wall of Death
A radio telescope is an instrument used to observe those parts of the universe normally hidden to us - that is the radio frequencies of the electromagnetic spectrum, not the inside of your mate's wallet in the local, which will ever be hidden to us.
Radio telescopes consist of a large parabolic dish that reflects and focuses radio frequencies onto a receiver mounted on an arm in the middle of the dish.
Some radio telescopes are huge constructions, the Jodrell bank telescope and the Arecibo dish in Puerto Rico for instance. Some are small but are part of a huge array spread over hundreds of miles to increase their sensitivity, the Karl G. Jansky Very Large Array (VLA) in New Mexico that consists of 27 dishes for example.
They all serve the same purpose, to collect radio signals from around the world so that they can be analysed and studied to understand the nature of that part of the universe we cannot see with the eye.