Level 1: Copy-editing, straight grammar, syntax, spelling, and ensuring your sentences make sense.
Level 2: Substantive-editing, tightening and clarifying at a chapter, scene, paragraph, and sentence level.
Level 3: Developmental-editing, mostly for large papers or stories, considers the flow, pacing, plot, and outline of the work as a whole, helping everything to intertwine together in a way that ensures the work is both readable and enjoyable.