its the aperture which is how much light the lens lets in to the sensor, so the higher the aperture or in your case the lower the number, the brighter the picture in low level lighting, it basically allows you a higher shutter speed in lower lighting condtions
this could be helping a lot my camer is a shitey wee digital thing but it has shutter speed 1/2000 which is quick but the photo is like always black so if i have a low number and high shutter speed that should help?