In a "rolling shutter" sensor, the start and end of exposure on each row or column or individual pixel happens sequentially, so not all the pixels are exposed at the same time. It can take up to 1/frame rate for all of the pixels on the sensor to become active. The effect will be noticed if the object is moving. See image below.
Rolling shutter sensors allow for a pixel to be designed with fewer transistors, reducing cost, and in some circumstances, increasing the well capacity and hence the quality. There are some modes of operation that can be implemented in rolling shutter cameras, such as a global start.
This is where all the pixels are turned on at the start of exposure, but are turned off in sequence. This can lead to a longer exposure at the bottom of the image, resulting in some blurring. However with a carefully designed system that uses a short exposure strobe light at the start of the sensor's exposure, it can yield good results in dark areas.