The resolution of a telescope of its ability to distinguish small details on an object (moon crater details), or the ability to distinguish closely located objects from each other (double stars).

The resolution of a telescope is directly proportional to its aperture and is measures in arcseconds.  An arcsecond is a 3600th of a degree.

A telescope with an aperture of 200mm will have a resolving power of 0.58 arcseconds.  The larger the aperture of a telescope is, the smaller objects can be resolved.

Looking upwards from earth though, the motion and instabilities in the Earth’s atmosphere do often limit the resolution to 1 arcsecond.  That is why large professional telescopes typically are being built on mountaintops, in areas with a very dry climate e.g. the European Extremely Large Telescope is not being built in Europe, but 3046m high in the Chilean Atacama desert, the driest desert in the world.