A robot doesn't know where it is. It uses sensor readings + thousands of hypothetical positions (particles) to figure out its location. Drive the robot and watch the particles converge.
How it works: The purple cloud is 500 particles — each one is a guess of where the robot might be.
As you drive (WASD), particles that match the robot's sensor readings survive; bad guesses die.
Eventually the cloud converges on the robot's true position. Add landmarks with Click. More landmarks = faster convergence.