Each basic engine function (such as ignition, fuelling and boost control) is governed by one or more 2-dimensional maps. Let's look at boost control as an example.
A typical boost control map is an 8 by 8 grid of numbers. The rows represent RPM, whilst the colums represent engine load. The values in this grid are the desired boost pressures. Many times per second, the ecu will check the current engine RPM and engine load, and will look up the boost pressure that it should try to maintain. In the example above, each row in the grid might represent 1000 RPM steps, the 8 steps being equivalent to 1000, 2000, 3000, 4000, 5000, 6000, 7000 & 8000 RPM.