The Coriolis force is what seems like a force to an object that is moving within a rotating frame of reference. As the object's motion appears curving when viewed from that frame of reference, it seems as if there is a sideways force. It is a factor in rotating fluids, e.g., atmospheres, stars, circumstellar disks, clouds in galaxies, etc. The apparent force is to the left if the rotation is clockwise, and to the right if it is counterclockwise. The apparent deflection of the object is called the Coriolis effect.
A beta plane is a planar approximation of a surface affected by the Coriolis force. It is used to model phenomena without all the complications of a spherical surface, and is presumably most useful for phenomena with a span much smaller than the size of the spherical surface