Generally, Level 1 represents business-as-usual, or no change from today. Level 4 represents a very large change from today: it may be questionable whether the highest levels are achievable or realistic, but they are included so you can explore how changes in different parts of the system interact, and what combinations of actions may be needed to reduce emissions. Level 2 and 3 then represent intermediate levels of change.
In some cases, the Levels instead represent alternative choices, which are not obviously ordered. Hover over the levels for each lever to see their description.