Odometry Drift
Wheel odometry adds up small wheel rotations to estimate the robot pose. Every measurement has a little error, and because the errors add up, the estimate drifts further from the truth over time.
What to notice
The navy path is the real robot. The red path is what the robot believes from its wheels alone. With only noise, the estimate wanders randomly. With a wheel size error, it bends the same way every lap, so the error grows steadily. Heading error is the worst kind, because a small angle error turns into a large position error the further the robot drives.
This is why real robots fix odometry with other sensors. A LiDAR matched against a map, GPS outdoors, or an IMU for heading can pull the estimate back. Combining them is the job of the Kalman filter and particle filter.