In confocal microscopy, scanning mirrors are used to direct the beam of light so that only one point in the sample is illuminated at a time. This beam is scanned horizontally across the sample to image the whole sample.
Slower scanning speeds create an image of higher resolution. However, if the scan speed is too low, the high levels of light exposure will cause the flurophores in the sample to undergo photobleaching. This means they are no longer able to fluoresce and so your sample can not be imaged.