Tell us more info, (is it hard drive or cd rom? or else? what brand and spec is it?)
Usually Dma means that the information will go directly to memory or requested source, using the controller chipset on itself or on MB, bypassing the CPU. So, it is faster (much faster for hard drives) and usues less cpu load during its operation.
(this is my understanding so if anyone sees mistake plz correct me ill appreciate it )
And most modern HD or CD or DVD or CDRW (from last 3 years) support DMA. so turn it on and see if they function correctly, if they dont then u can always goto safe mode to change the settings back, good luck.