Lookup Tables
The PCM needs a lot of lookup tables to find the right numbers to use in fuel and timing calculations. Lookup tables are kind of like a translation dictionary. The "input" to the translation dictionary is the English word, and the output is that word translated into another language. The input and output may not look at all alike, but they are definitely related by some rule. The input to a PCM lookup table is always the signal from an engine sensor, and the output always controls some aspect of engine operation, like injector pulsewidth or ignition timing.
One-dimensional tables use the value of one sensor input, such as ECT, to find the right number, such as power enrichment, for some calculation. My kPa to vacuum conversion table at the top of the Units page could be thought of as a one-dimensional lookup table with kPa as the input and vacuum as the output. But kPa and vacuum have a fixed mathematical relationship - meaning that you can get from one to the other with a formula that works for all values. The PCM uses tables when it can't use the same math formula to get from all of the inputs to their corresponding outputs.
Two dimensional tables use two sensor inputs, such as MAP and RPM, to point to the right number, such as VE, for some calculation. A two dimensional table is like a spreadsheet - the two inputs choose the particular row and column, and the value in the cell corresponding to that row-and-column pair is the output.