Via Quartz:

“The phase speed is infinite—much larger, infinitely larger than the speed of light,” Mazur tells Quartz.

This doesn’t mean light itself is traveling faster than the speed of light, which would violate the laws of relativity. “Phase velocity” refers to the speed of the crest of waves that ripple out when light strikes a material. The Harvard scientists created a material that allows these wave crests to move infinitely fast. This is a strange thought to wrap your head around, and means the crests of the waves are oscillating through time, but not space. Under these peculiar conditions, the Harvard scientists found that it’s easy to manipulate the photons, squeezing them down to the microscopic scale and turning them around. In other words, we can treat photons in the same way we currently manipulate electrons.