A difficulty in a Difficulty Chart Obby (DCO) is an ordered level of stages that share a similar skill expectation, or, put simply, a similar difficulty. Charts stack those difficulties from the easiest (often Effortless) through the hardest difficulty the creator supports (sometimes Impossible or Catastrophic or a custom difficulty the creator picked).
Why charts use difficulties
Difficulties give players a predictable sense of progression. Instead of one flat difficulty curve, a chart telegraphs how precise a players required movement, timing, and skill are about to become. They convey this in simple and understandable words, so every player both new and experienced can instantly tell how hard a difficulty is.
Wraps and terminology
Players often describe wraps (moving around the outside of a part), flicks (quick camera or jump corrections), and head hitters (jumping so your avatar's head interacts with geometry). Those mechanics show up more often as you climb the chart.