Shutter speed is the time that the shutter stays open while capturing the image. Your camera, if it has a shutter speed option, will display fast shutter speeds in fractional seconds. For example, 1/250 means that the shutter will stay open for 1/250th of a second. The speeds will usually go into low whole numbers, like 1s. At larger fractions like 1/5th of a second, your camera settings may convert to decimal and display as .2s.
The longer the shutter stays open the more time light has to be absorbed into the image. So long shutter speeds, which, in terms of cameras today, is anything longer than 1/30th of a second, will result in brighter images.
Changing shutter speed to adjust light has side effects. At longer speeds, motion will begin to blur your image, and even the slightest shake from pressing down a shutter button can cause unwanted blur at these sensitivities. Long exposure photographs, which are the nice images of city traffic becoming streaks of light, are long shutter speed, tripod mounted, remote activated shots. When you see these images, the primary manual setting used is shutter speed.
At the other end of the spectrum, for any fast subjects such as animals or athletes, shooting at slow shutter speeds will only result in that same incoherent light blur. While this is great if this blur is intentional, nothing is worse than getting that perfect high-action shot only to see that the photo is just one streak of light where that cheetah should be. Getting the timing right for these shots is hard enough, so don’t be conservative with the the setting. If you don’t know, go as fast as you can while getting the right amount of light. You may even want to adjust other settings like ISO so you can push the shutter even faster.
So now you know, if you are shooting your cell in low light situations vs in sports mode, the difference is shutter speed. Be sure to hold your phone extra still in the dark!