This describes a high-speed method of accessing the contents of memory chips in your computer. If a video card wants to read information from the main memory it first has to ask the central processor chip. The central processor then accesses the memory location and passes the information back to the video card. In a DMA scenario, the video card temporarily takes over control of the computer from the central processor and directly retrieves the information from the memory.
In the first case, the processor has to stop what it's doing and service the request from the video card. In the second case, the processor can carry out its calculations without interruption. In short, using DMA means that data can be sent around the computer much faster and that the central processor can carry out its calculations in a shorter time.
To support DMA, you need a PC that's designed with a central bus (the electronic connections that link all the components together) that can support an intelligent video card master - which in DMA talk is called a bus - because it can directly control the memory and retrieve data over the bus.
