It's the frequency at which the backlight of the panel operates at.
Higher frequency (measured in Hertz usually or HZ), backlights can reduce perceived stuttering from low frequency sources.
It's closely related to frame rate and actual panel refresh rate.
From my understanding:
Think of the pixels of the panel and the backlight as 2 separate pieces (which they are in typical panel types)
For content displayed at differing frame rates on the pixel part of the panel, a higher frequency backlight makes more sense.
If the content on the picture is moving quickly or is of a high frame rate, a higher frequency backlight will create an effect of smoothing the video. The content can't obviously switch frame rates to match the panel and...