What Nil means mechanically varies by game, but commonly it is a very difficulty part of whatever game it is in.
What it feels like
Nil stages can be long, cruel, and psychologically taxing: not because randomness hates you, but because the margins are intentionally tiny and the punishment is intentionally heavy. Clears are rare enough that communities celebrate them.
Typical obstacles
Because Nil is game-specific, the obstacles are less standardized than mid-chart difficulties. You might see the hardest wraps in the project, the longest gauntlets, or jumps that assume mastery of niche techniques, always within that game's rules.
Where it sits on the chart
Nil sits after Absurd on charts that include both. If a game stops earlier, you might see Nil used as the final band instead. Either way, treat it as the summit unless the creator explicitly adds beyond-Nil content.