Refresh rate is a measure of the number of times a display's image is 'repainted' per second. The refresh rate is measured in hertz. A display's refresh rate depends on the video card used (although displays do have a maximum refresh rate).
The original standard for refresh rates, developed by the Video Electronics Standards Association (VESA), was 60 Hz. At 60 Hz the display's image flickers noticeably which can cause eye fatigue and headaches. A new standard set the refresh rate to 75 Hz. IIRC, under European Law it is illegal for an employer to force an employee to use a display with a refresh rate of less than 70 Hz.
DRAM modules also have a refresh rate. A DRAM module is made up of electrical cells which must be refreshed thousands of times per second to prevent them losing data. Some DRAM modules are able to refresh themselves independently of the processor. These types of DRAM are commonly used in laptops due to their lower power consumption.