What is difference in link rate and pixel rate in Display Port
It's no different than having a gigabit ethernet link between devices, and sending only the data packets you really need at some slower rate, like at kilobit per second.
In some other display interfaces, the pixel clock is used for the link clock to transmit pixel data over the link. It also means that for each or any possible pixel clock rate, you must support many different link clock rates.
But in DisplayPort, there are only few fixed link rates available, and the pixels are sent in packets, along with timing info how to regenerate that pixel clock in the receiving end.