When purchasing telephoto lenses, the term crop factor often comes up. This describes the length ratio of a sensor image diagonal compared to the diagonal of the 35mm/full format (36x24mm). The format has historical reasons, when still on negative film was processed, whose images corresponded to the full format.
With the development of digital sensors, these were almost always smaller for cost reasons. However, manufacturers wanted customers to be able to keep using their lenses. If the same lens is now used once on a full-format camera and once on a camera with a smaller chip, the focal length does not change, but only a section of the original full-format image can be seen. The ratio of the two image diagonals is called the crop-factor.
If the crop-factor is multiplied by the focal length of this lens, the result is the focal length of a lens that, when mounted on a full-frame camera, produces the same image detail as when capturing with a small sensor and unchanged lens.
Marten2k, CC BY-SA 3.0, via Wikimedia Commons (translated)