It means that 1866 only works when you install ram in ram slots 3 and 4, not 1 or 2. This is a limitation of the motherboard chipset. I recommend the following brands of ram in 1333 or 1600, whichever is a better value: crucial, corsair, samsung, kingston, pny, pqi or gskill. Ram labeled as "value" is less likely to overclock. You may also check gigabytes website for tested ram modules. Sorry I can't post a link.