It refers to the way the circuit board is designed. Older keyboards were generally designed with the intention that only one, maybe two, keys would be held down

It refers to the way the circuit board is designed. Older keyboards were generally designed with the intention that only one, maybe two, keys would be held down at once. But, modern applications (especially games), often require or support many keys to be pressed at once. This has required keyboard circuits to be designed to handle that. Ghosting is what happens when it's not designed to handle that (the computer registers key presses that weren't actually pressed) and N_Key Rollover is what you get when you eliminate ghosting and allow N number of keys to be pressed at once.