Introduction to Shutter Priority (S/Tv) mode
What the shutter speed number means
Shutter speed is the length of time used for the exposure. 1/1000 second is a very short time; 1/15 second is much longer. A fast shutter can freeze movement. A slow shutter records movement for longer, so a moving subject or moving camera can blur.
Where to change it
On most cameras you can control shutter speed in Shutter Priority, marked S or Tv, or in Manual mode, marked M. One of the command dials normally changes the value. If the number will not change, check the shooting mode first. Phones often hide direct shutter control unless they have a Pro or Manual mode.
What else changes
If you choose a faster shutter speed, less light is recorded during the exposure. The camera may compensate with a wider aperture or higher ISO. A slower speed records light for longer. This is why shutter speed cannot be considered completely separately from exposure.