Unity's PhysX is a powerful physics engine integrated into the Unity game development platform. It enables developers to create realistic simulations of physical interactions, such as collisions, gravity, and fluid dynamics. This allows for more immersive gameplay experiences, as objects in the game world behave in a way that players expect from the real world. The engine supports various features, including rigid body dynamics, soft body physics, and character controllers. By utilizing PhysX, developers can enhance their games with lifelike movements and interactions, making it easier to design complex environments and engaging gameplay mechanics.