A refractor telescope, also called refracting telescope uses a simple straightforward design that is virtually mainenance and adjustement free. The telescope uses a lens as its objective to form an image.
Think of a typical spyglass you see in pirate movies. That’s essentially what a refractor telescope looks like. The refracting telescope is essentially a long tube with one big end and one small end. There is a larger glass lens on the bigger end which captures and bends the light to focus it down the tube into a tiny point at the small end. Cheaper refractor telescopes often use a plastic lens instead of glass. The refracting telescope design is also used in other devices such as binoculars and long or telephoto camera lenses.
Galileo used the refracting telescope concept to create what is commonly called a Galilean telescope which uses a convex objective lens (big end) and a concave eyepiece lens (small end). Galilean telescopes produce upright images whereas certain types of telescopes provide inverted images, sometimes both upwards and sideways.
One of the issues with this design is the small aperture of the refractor telescope. In order to increase the aperture size significantly, the instrument would require additional lenght which becomes problematic in terms of manifacturing cost and portabilty.