Most people who use flash understand that your sync speed is the maximum shutter speed with which you can use flash. What people generally don’t know is why.
“My camera goes all the way up to 1/8000th of a second, so why can’t I use flash past 1/250th?”
Well, this is really all down to how your shutter works in a DSLR. The types of shutters in DSLRs are called “focal-plane shutters”.