"fly-by" DMA?

I don’t understand what “fly-by” DMA means. What is it, and what are its advantages and disadvantages?

“Fly-by” DMA refers to the normal peripheral/memory transfer in which the data flows into/out of the memory using address lines for only one side of the transfer. “Memory to memory” DMA is the other type, for which both source and destination address need to be specified. Fly-by DMA is much faster because memory-to-memory DMA results in two copies – one from the source to context RAM and the other from context RAM to the destination.