ok, well, when you overclock you CPU by upping the FSB (oh dear is already sounding rly jargonny lol

), the ram also gets overclocked.. this can cause a problem if your ram is not super amazing and doesnt like to run at this high speed...
so some motherboards offer dividers, 5:4 for example, would mean for every 5mhz the CPU fsb goes up the ram goes up by 4.. this lowers overall performance slightly but can help you overclock if your ram is not up to scratch
1:1 means no divider!