The frequency at which the LED's operate. LED's dim by what is known as Pulse-Width Modulation or PWM. Basically, to get an LED to dim, it simply strobes faster than the eye can see (although some people can actually see it, like flickering on a computer monitor at 60 Hz).
Cameras pick up on this though since they operate at a much lower frame rate than our eyes do. So in order to make a LED flicker free, the answer is simply strobe it even faster so the camera can't tell. This costs more money however which is why flicker free stuff costs more.
Jeffery is partly correct in saying there is nothing you can do with the fixture. However, depending on the camera, it may have a setting that doesn't pick up on the flickering LEDs. You can try a higher frame rate like 30 or 60 frames per second for example. This has other effects though that might not be desirably depending on the effect you are looking for with the camera. Some higher end cameras also can have a setting generally built into the CCU that can fix the issue as well.