A fuel map could for instance be a type of engine map and serves as a sort of guideline that an engine will attempt to follow when given specific commands. In the case of the fuel map for instance, depending on the type of engine and its design, you may have a XYZ table with the x axis having a speed range of say 700 to 2000 rpm, a Y axis with net torque values ranging from 0 to 800Nm and a Z axis of fueling values.

in the simplest form, during the calibration process, you tune the fueling values in the table at  say a given XY point say 2000 rpm and 800 NM and repeat the process across the boards to complete the table. You may for instance keep the engine at 1000 rpm  and then target each torque in the Y axis and determine a fueling value. and repeat the process across the table till you have a complete XYZ table.

When you need to run the engine at a specific point in the future, the engine will reference this table in you calibration and provide the fueling required based on the values in your XYZ table. In situations where you do not have an exact match, the engine will try to iterate between two points to provide the required fueling you need.

Hope this answers your question. Sorry its not a concise answer but hope it helps.