Plain English Meaning
When a spinning ball or cylinder moves through the air, it gets pushed sideways because the spin makes the air flow faster on one side than the other. The faster side has lower pressure, so the object is pulled toward that side. This same idea — air moving faster on one side than the other producing a pressure difference — is used to help explain how a wing creates lift.
Full Definition
The Magnus Effect is the sideways force generated on a spinning cylinder or sphere moving through a fluid. The spin drags air faster around one side and slower around the other, producing lower pressure on the fast side and higher pressure on the slow side. The pressure difference creates a force perpendicular to the direction of motion. In aerodynamics, the Magnus Effect is used to illustrate how circulation around a body produces lift, which helps explain how a wing generates lift through the circulation of air around it.