A patch is a type of deformable object. A patch object is useful for creating gently curved surfaces, and provides very detailed control for manipulating complex geometry.
Example of a patch model
When you apply an Edit Patch modifier to an object or convert it to an editable patch object, 3ds Max converts the object's geometry into a collection of separate Bezier patches. Each patch is made up of three or four vertices connected by edges, defining a surface. Patches also have interior vertices that you can control, or let 3ds Max control for you.
You control a patch surface's shape by manipulating the vertices and edges. The surface is the renderable geometry of the object.