The sensitivity of a telescope is the smallest signal that it can clearly measure from a source in space. It is the minimum brightness that a telescope can detect. A telescope with high sensitivity can detect very dim objects, whereas a low sensitivity telescope can only detect the brighter objects in space. Many objects in space are very dim as seen from the Earth. Some are naturally dim objects because they do not emit much light. Others only appear to be dim because they are a great distance from us. It is important for a telescope to have the greatest sensitivity possible, so that it can observe the many different types of objects in space.
A telescope's sensitivity depends on how much light or electromagnetic radiation it can collect. This depends on the surface area of its lens, mirror or dish (for radio telescopes), and the pixel size used in a telscope's detectors. The larger these are, the higher the telescope's sensitivity will be. Longer exposures by a telescope's cameras will also increase the sensitivity of an astronomical image.
Images of the Crab Nebula taken using a telescope with lower (left) and higher (right) sensitivity.