In principal, shutter speed is pretty easy to understand: it is the amount of time light is allowed to shine on the sensor or piece of film. The most-used shutter speeds are fractions of a second, like 1/30 or 1/500, but the range is huge. Many cameras will allow you to dial in 1/8000 of a second, and if you know how, you can use shutter speeds of many minutes or even hours.
Every time you double the shutter speed, you double the amount of light that reaches the sensor, and when you cut the shutter speed in half, you cut the amount of light in half.
That all seems pretty straightforward, but you might be surprised by how many photographs are ruined or missed entirely by bad shutter speed selection.
The “magic bullet” shutter speed for freezing action photos of people tends to hover around 1/1000 of a second. That’s one millisecond for you math and science nerds. There are very few human activities that move so fast they are blurry at 1/1000 of a second. The trouble is that such a short amount of time is an equally small amount of light, so to get such a fast shutter speed requires some combination of large lens aperture, higher ISO values and plentiful illumination, like daylight.
The other end of the shutter speed spectrum is long shutter speeds. In terms of human movement, people playing sports start to get blurry at about 1/250 of a second, and people doing everyday things get blurry at about 1/30 of a second. It also becomes more difficult for human hands to hold a camera steady at about 1/30 of a second, though various camera features can stabilize a hand-held camera down to somewhat slower shutter speeds like 1/8 of a second.
By the time our shutter speeds get to one full second, a lot of motion takes place in a photograph, and we can take advantage of that as well, in the form of motion blur to express the movement in a scene, or as an artistic enhancement.
Being able to fine tune shutter speed in-camera is a real game-changer, and I urge you to explore and experiment with it, and have fun.