The horizontal reference planes in Revit - typically one per building story - that floors, walls, roofs, and nearly every other element attach to and measure from. Levels are one of the very first things set up in any Revit project, and almost everything else depends on them: worksets, phasing, schedules, and family placement all reference a level, directly or indirectly. Getting levels wrong early on is one of the most common causes of frustrating rework later.