The shutter lag refers to the duration that you have to wait from the time that you fully hold down the shutter button of your camera until the time that the photos are actually recorded by the image sensor. Different models have different shutter lags.
Although commonly, the lag time accounts for a fraction of one second, it is still enough to get you miss a spontaneous image at times when the subject is out of the frame. The shutter lag becomes more serious in some of the following cases: when the subject is in the long distance from the photographers, when you fully extend the long optical zoom lens, or when there is a lack of light. The solutions for this lag problem is to lock the focus to make the autofocus process become faster through holding down shutter button halfway.