I've heard the term 'DMA' being used a lot in reference to older consoles. My very basic understanding is that it's a way for the console to access memory directly. But directly as apposed to what? isn't RAM always accessed directly?
"The console" (or other computer) is made up of various parts, including the processor, the memory, and peripherals such as video display controllers, I/O chips to read from a keyboard or joystick, disk controllers, and so on.
When using "programmed I/O," the CPU reads data from or writes data to a peripheral and, if the data need to be stored in memory, the CPU does that as well. So the CPU might read a byte from the disk controller, store that byte into a buffer in memory, and repeat that to read an entire sector from disk.
DMA allows the CPU to be paused and/or disconnected from the memory bus so that peripherals can directly read data from or write data to the memory without the involvement of the CPU. Thus, instead of the example given above, a disk controller could write its bytes directly to memory while the CPU waits, which is faster because rather than "read byte then write byte" by the CPU there's just "write byte" by the peripheral.
(Too long for a comment)
DMA is not in any way console or retro computing related. It is actual technology and today used more than ever. Each of your disk reads and writes, each transfer to your graphics or network card will usually involve DMA.
DMA describes the access of components, other than the CPU, on (a CPU's) memory. In general it's the ability to run a multi master bus system. Further information can be found at the corresponding Wiki page and next to endless other resources on the web.
DMA is a technique where you can move memory around without involving the main CPU. This was a huge win for old gaming hardware as an awful lot of time can be spent simply shuffling graphics data around the screen memory. Offloading this task to custom silicon meant the CPU had more time to do much more interesting and computationally expensive things. The actual copying itself can be done much more quickly too, meaning smoother scrolling etc.