A low power wide field telescope that's attached parallel to the main telescope. Its purpose is to initially locate an object or a section of sky where the object can be found. When the finderscope is perfectly aligned with the main telescope, the object once centred in the finderscope will also be centered in the main telescope field of view.
Finderscopes are usually small refractors such as 10x50mm or 6x30mm models. Since they have a field of view spanning several degrees of sky they are ideal for locating objects or the correct region of sky.