They change the frequency of the data passed from the Gpu and the memory of the graphics card.
The faster you can clock a device the more data you can process from the memory of the device or a Gpu in less time.

The clock controls the switching speed of all the transistors in the memory or Gpu.
So when some one talks about frequency, it is the rate the transistor is switching.
Faster switching in less time = More data processed. In one clock cycle.