Calipers enable measurements to be made.
In most machines the calipers are placed and then moved via a mouse or tracker ball. Usually a measure is given numerically on the screen. In general front to back measures of circular structures are better, (e.g. vessels) as edge artefact can cause loss of definition at the edges.
In this image of the abdominal aorta the calipers have been placed at the front and rear (i.e. anterior-posterior (AP) measurement) and the figure of 65.3 mm is displayed as the AP diameter.