Shutter speed refers to the amount of time it takes for you camera to capture the photo. For example 1/400 refers to 400th of a second. When going below 1/80 of a second you must have a tripod to avoid camera shake this cause unwanted blur. You can create long exposures by leaving it open for up to 30 seconds or if you have a shutter release cable you can leave it open for minutes to hours. This is commonly used for night time photography to capture the night sky, create light painting, and to have smooth water in your landscapes.
A fast shutter speed lets in less light, but gains a sharper photo, as you’re only taking a snapshot of a fraction of a second (commonly used in sports photography).
A slower shutter speed lets in more light, and creates movement blur as you will be recording movement over a longer time frame (used to make landscapes with smooth water).