In 1835 French engineer-mathematician Gustave-Gaspard Coriolis described a force which acts at right angles to the ordinary Newtonian laws of motion when they are used in a rotating frame of reference. The effect of the Coriolis force is an apparent deflection of the path of an object. The object does not actually deviate from the path but appears to because of the motion of the co-ordinate system.
From the video, consider whether the ball is travelling in a straight line, or following a curved path. The answer is that it depends on your frame of reference. When you are within the ‘world’ that is the playground, the ball appears to travel in a straight line. But when the view changes such that your world is the roundabout, the ball appears to follows a curved path. Coriolis is seen to act sideways on something travelling in a straight line, when the world that contains that motion is itself rotating.