Rainbow Obby is an obstacle course — an 'obby', in Roblox terminology — built as a colourful gauntlet of jumps, narrow beams, moving platforms, spinning blades and disappearing tiles. You run, jump and time your way from checkpoint to checkpoint toward the end.
The format came out of Roblox and has spread everywhere because it's endlessly readable: you can see exactly what killed you, and exactly what you should have done instead. Checkpoints keep the frustration manageable.
Short for 'obstacle course' — a Roblox term for a level made of jumps, moving platforms and traps that you navigate to the end, usually with checkpoints along the way.
Yes — falling returns you to the last checkpoint rather than the start, which is what keeps the format playable.
Yes — it runs in the browser with nothing to install.