Essentially, the full image frame as captured by the film or the digital camera sensor, usually in a tall 4:3 aspect ratio before being cropped down to a cinematic wide screen format (1.85:1, 2.35:1 etc.). So it shows off much more vertical space. To be fair, the director and cinematographer would compose the shots for the target aspect ratio, so the extra vertical space is usually visually uninteresting (sky, ground) because all the production design and composition will be set up for the frame they wanted to shoot. But purists are always after it for preservation reasons.

Also, of note to this discussion, it is typically very lightly compressed as it is the version intended to be used for post production and editing so is in the highest available quality.