If you have ever browsed studio strobes or even own a speedlite with the feature, you may have come across that term in its title, “high speed sync” or “hss”. Let’s take a minute to help you understand what that is and how to use it to your advantage.

Every camera has its own maximum sync speed, or the fastest shutter speed you can use while shooting an image with flash. If you exceed this shutter speed while shooting with flash, you will see a horizontal black bar covering up part or all of your image. When you take a photo, the first part of your shutter (first curtain) separates from the second, the sensor gets exposed for the amount of time that your shutter speed is set to, and the second curtain closes behind it. 
When you add flash to the mix, it can only properly expose the image up to your camera’s maximum sync speed. What happens then is you press the shutter release button, the first curtain opens, the flash then fires, and the second curtain closes. If your shutter speed is set too high (past the maximum synchronization speed), the second curtain will actually block part of the sensor as the flash is exposing the image. With high speed sync turned on, the flash pulses multiple times, exposing different parts of the image as the curtains fall. It happens too quickly for the human eye to see but the effect is surely noticeable.
You might be wondering how this happens if there are other lights in the room or scene. The unique thing about using flash photography is that it operates completely separately from continuous or ambient lighting. The two factors to keep in mind are that shutter speed affects only the ambient or continuous lighting in the exposure and your aperture affects the flash output. Experiment using different combinations and settings to find out what effects work best for your taste or needs. 
You are able to combine continuous light and strobe lights for particular effects. A great photographer to cite as an example is Nick Fancher. He is a master of using various combinations of gels, continuous lighting, and strobe lighting to create amazing works of art. 
As this was just a general overview of what it is and how it works, do some extra research to master this technique and create portfolio-quality work. Keep on shooting!