Shutter speed refers to how long your camera is open for - the speed of the shutter. It is used to capture movement, by either freezing it still or by showing movement with blur.
When shooting without a tripod (which for us is all the time) we try not to let our shutter speed get below 60/s as it can create blur unintentionally, just by not having still enough hands. Try and keep your shutter speed above at least 125/s when holding the camera or shooting a moving subject, this will avoid any unintentional movement or blur in your images.
If the shutter speed is getting quite slow and you can’t change your f stop to let anymore light in, this is when it is time to bump up your ISO.
This is a video that explains shutter speed really well too (no daggy guys this time, just daggy graphics).