The aperture is one of a telescope's most important characteristics. The bigger the aperture the more light the telescope collects and the higher the resolution. The latter means that the telescope displays finer details.
However the greater the aperture the more sensitive the telescope is to the Earth's air turbulence. A large telescope can only display its strengths under a good sky.
The aperture is given in millmetres. Since many telescopes originally came from the USA, this information is often given in inches. In this case 1 inch is equivalent to 25.4 millimetres.
A telescope with an aperture of 200 to 205 millimetres would be designated as an 8 inch, and so on.