The word "level" in Imperialism refers mostly to the development of terrain. Most land tiles can be developed beyond their initial resource yield. Levels are numbered "I", "II", and "III" in the Technology screen but the simpler "1", "2", and "3" elsewhere. Our pages use the simpler numbering.
The details of levels can be seen in the descriptions of the civilian units in the university window. A civilian can improve a specific tile for a particular yield. Most add one load of resource per level, but for coal and iron a miner adds two.