A 'pakset' is the data set that Simutrans needs to run: it contains all the data for the graphics of individual items (vehicles, roads, railways, buildings, etc.) and all of the balancing information (such as prices, speeds, capacities and so forth) necessary to make the game complete.
When Simutrans could only support 64px size graphics, pak128 already started. First pak to feature a complex economy and have a very wide variety of objects. It contains roughly 7 time more graphics data than pak64 and thus requires by far the largest amount of RAM and processing power of all Simutrans sets.