focal length

In general terms, this is the distance between a lens or mirror and the image it produces. The longer the focal length, the larger the image. Typically, astronomical telescopes have focal lengths in the range of 300 mm for the smallest up to maybe 4 metres or more for a large amateur instrument. But between 600 mm and 2000 mm is commonplace.

If you have two telescopes of the same aperture, but differing only in focal length, the longer one will give a higher magnification with any chosen eyepiece. But magnification isn’t everything. You may want a telescope with a wide field of view, such as for observing large deep-sky objects or comets, in which case the shorter focal length would be preferable, as well as being more portable. Alternatively, for observing planets, the longer focal length would be better.  In addition, the shorter the focal length, the brighter the basic image it provides.

The aperture and focal length are linked by the term focal ratio, also known as f-number.

Long and short focal length compared
Two lenses with the same aperture but different focal lengths. The longer focal length gives a larger but dimmer image