What means DDR dual channel?

In DDR single channel mode , computer data will access DDR DIMM1 first; when data over DIMM1 size then data will access DDR DIMM2. DIMM1 will access more often than DIMM2, Suppose be able to find the DIMM slot configuration in MB manual . In DDR dual channel mode, computer data will access 2 DIMMs at the same time because there are 2 DIMMs with the same size and speed . Usually the enhanced performance of DDR dual channel can't shown up with small data transmission. But users can feel higher performance obviously with lagrge data transmission.