Well basically what the others said but in a little more detail
A hybrid is a cros between two different species (Such as the cross between lions and tigers to make the two hybrid species ligers and tions) Unfortunately this crossing of species often leads to a lot of health problems.
Campbell's Russian Dwarfs and Winter White Russian Dwarfs are the only different species of hamster which can breed together successfully because both have the same number of chromosomes unlike the other hamster species. The hybrid of these two is called the Russian Hybrid.
Because Syrians can only breed successfully with other Syrians they cannot become hybrids.