bus_dma (9). What exactly means "Loading of memory allocation" ?

[Alexej Sokolov]

Hi,
at first the cut of text from man (9) bus_dma:
bus_dmamap_t
             A machine-dependent opaque type describing an individual
mapping.
             One map is used for each memory allocation that will be loaded.
             Maps can be reused once they have been unloaded...

Question: What exactly means "Loading of memory allocation" in this context
?
Could anyone explain it or give me some little example with DMA functions
for understanding it.

Thanks a lot,
Alexej