The Redis Lua Debugger, or rld, is a tool that helps developers debug their Redis Lua scripts more efficiently. It's written in Lua and runs directly on the Redis server, requiring only the script and Redis to use it. The debugger can be activated by calling its `start` function at the beginning of the script, allowing it to track execution and print information about the script's output. This results in a better understanding of what went wrong during execution, making debugging more efficient and effective.