This is a technical term referring to the way the image sensor is sensed. If the sensor uses a rolling shutter, it means that the image is scanned in rows from one side of the sensor to the other (usually from top to bottom, center to edges, or other ways). This type of scanning is typical of sCMOS sensors. Individual pixels are not read out at a time, the disadvantage of this scanning method is smearing.
Sensor readout options:
