Refresh rate is the number of times per second the display grabs a new image from the GPU. It also decides the length of latency between each image. Higher refresh rate means less latency between frames.
For a display with 30 Hz: 1 Hz = 1000 milliseconds, 1000 milliseconds / 30 Hz = 33.33 milliseconds = the latency
Latency due to limitations of refresh rate can be reduced by increasing the refresh rate. For example, a display with 60 Hz would have the latency of 16.67 ms, half of the 30 Hz display.