From Team 449 Wiki
Mecanum wheels are wheels with rollers mounted around the edge. The rollers' axis of rotation is angled 45°, allowing a mecanum drive to have omnidirectional motion.
How they work
Main article: Mecanum drive
Due to the angle of the rollers, the wheels create a resultant force at an angle to plane of the wheels, in contrast to normal wheels, which have a force parallel to the plane of the wheels. When all wheels move in the same direction at the same speed, the sideways components cancel out, and the robot moves parallel to the plane of the wheels. By adjusting the speeds of the wheels, the robot can be made to move omnidirectonally.