A “cropped sensor” usually means an image sensor that is smaller than a 35mm full-frame sensor (36×24mm). Because the sensor is smaller, it records a smaller central portion of the image circle projected by the lens. In that sense, the image is like a crop taken from what the same lens would show on a full-frame camera.

So the lens is not changing focal length, but the field of view becomes narrower because less of the projected image is being captured.

For example, Canon APS-C sensors are smaller than full frame, so with the same lens and camera position, you’ll see a tighter framing than on a full-frame body.

The term is common, though somewhat relative: “full frame” is based on the old 35mm film format, and larger formats also exist. In practical use, “crop sensor” just means smaller than full frame, with a correspondingly narrower field of view from the same lens.