The way I used to explain this on training courses was to get delegates to draw a simple graph, 2 dimensional, base line for speed and vertical line for Revs. now to visualise picking the line drawing up at various points to look like a range of hills, that represents 'Load', when they asked what load was think cruising down the motorway on very light throttle at 3,000 RPM, you meet a long hill and the throttle has to be opened to maintain the same revs and speed, this is due to an increase in load or if you like work that the engine has to do in the same time. Not strictly true to think of load as work but close.
Your EC measures many things to calculate load and the map they talk of is a 3 dimensional graph that is programmed to give a specific amount of fuel at a range of speeds and loads, the amount required in the 2 circumstances above are different of course. Manufacturers have to meet strict emission regulations so are restricted in what they can do, the aftermarket often offers a 're-map' to give more performance or economy (sometimes both) that a manufacturer is not allowed to do in production. Claims are often exaggerated but it has to be said that most find improvements either in performance or driveability. Hope that helps?
